Remove numerous things deprecated during the 0.14 release cycle

Basically everything that has a direct replacement or no uses left.

Notable omissions:

- std.ArrayHashMap: Too much fallout, needs a separate cleanup.
- std.debug.runtime_safety: Too much fallout.
- std.heap.GeneralPurposeAllocator: Lots of references to it remain, not
  a simple find and replace as "debug allocator" is not equivalent to
  "general purpose allocator".
- std.io.Reader: Is being reworked at the moment.
- std.unicode.utf8Decode(): No replacement, needs a new API first.
- Manifest backwards compat options: Removal would break test data used
  by TestFetchBuilder.
- panic handler needs to be a namespace: Many tests still rely on it
  being a function, needs a separate cleanup.

test/c_abi/main.zig

@@ -5763,7 +5763,7 @@ test "f128 f128 struct" {
 }
 
 // The stdcall attribute on C functions is ignored when compiled on non-x86
-const stdcall_callconv: std.builtin.CallingConvention = if (builtin.cpu.arch == .x86) .Stdcall else .C;
+const stdcall_callconv: std.builtin.CallingConvention = if (builtin.cpu.arch == .x86) .{ .x86_stdcall = .{} } else .c;
 
 extern fn stdcall_scalars(i8, i16, i32, f32, f64) callconv(stdcall_callconv) void;
 test "Stdcall ABI scalars" {