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/priority_queue.zig

@@ -1,8 +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.
 const std = @import("std.zig");
 const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;