Remove a bunch of unnecessary includes.

This file contains nothing but no-op functions. There's nothing to include.

Change-Id: I3a21207d6a47fab3a00c3f72011abef850ed7b27
Reviewed-on: https://boringssl-review.googlesource.com/8541
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/thread.c

@@ -56,18 +56,6 @@
 
 #include <openssl/thread.h>
 
-#include <string.h>
-
-#if !defined(OPENSSL_WINDOWS)
-#include <errno.h>
-#else
-OPENSSL_MSVC_PRAGMA(warning(push, 3))
-#include <windows.h>
-OPENSSL_MSVC_PRAGMA(warning(pop))
-#endif
-
-#include <openssl/mem.h>
-
 
 int CRYPTO_num_locks(void) { return 1; }