remove redundant license headers from zig standard library

We already have a LICENSE file that covers the Zig Standard Library. We
no longer need to remind everyone that the license is MIT in every single
file.

Previously this was introduced to clarify the situation for a fork of
Zig that made Zig's LICENSE file harder to find, and replaced it with
their own license that required annual payments to their company.
However that fork now appears to be dead. So there is no need to
reinforce the copyright notice in every single file.

lib/std/Thread/AutoResetEvent.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! Similar to `StaticResetEvent` but on `set()` it also (atomically) does `reset()`.
 //! Unlike StaticResetEvent, `wait()` can only be called by one thread (MPSC-like).
 //!

lib/std/Thread/Condition.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! A condition provides a way for a kernel thread to block until it is signaled
 //! to wake up. Spurious wakeups are possible.
 //! This API supports static initialization and does not require deinitialization.

lib/std/Thread/Futex.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! Futex is a mechanism used to block (`wait`) and unblock (`wake`) threads using a 32bit memory address as hints.
 //! Blocking a thread is acknowledged only if the 32bit memory address is equal to a given value.
 //! This check helps avoid block/unblock deadlocks which occur if a `wake()` happens before a `wait()`.

lib/std/Thread/Mutex.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! Lock may be held only once. If the same thread tries to acquire
 //! the same mutex twice, it deadlocks.  This type supports static
 //! initialization and is at most `@sizeOf(usize)` in size.  When an

lib/std/Thread/ResetEvent.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! A thread-safe resource which supports blocking until signaled.
 //! This API is for kernel threads, not evented I/O.
 //! This API requires being initialized at runtime, and initialization

lib/std/Thread/RwLock.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! A lock that supports one writer or many readers.
 //! This API is for kernel threads, not evented I/O.
 //! This API requires being initialized at runtime, and initialization

lib/std/Thread/Semaphore.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! A semaphore is an unsigned integer that blocks the kernel thread if
 //! the number would become negative.
 //! This API supports static initialization and does not require deinitialization.

lib/std/Thread/StaticResetEvent.zig

@@ -1,9 +1,3 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-
 //! A thread-safe resource which supports blocking until signaled.
 //! This API is for kernel threads, not evented I/O.
 //! This API is statically initializable. It cannot fail to be initialized