[libc] Remove unused _Thread_local define from headers. (#156967)

It was added in dd33f9c to describe
thread-local errno, but is no longer used in the codebase (with the
exception of a single integration test, but the llvm-libc-provided
`#define _Thread_local thread_local` is not needed there anyway, since
`_Thread_local` is a keyword from C11 onwards.

Author: vonosmas

libc/include/__llvm-libc-common.h

@@ -37,9 +37,6 @@
 #undef _Alignof
 #define _Alignof alignof
 
-#undef _Thread_local
-#define _Thread_local thread_local
-
 #undef __NOEXCEPT
 #if __cplusplus >= 201103L
 #define __NOEXCEPT noexcept