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

It was added in dd33f9cdef9f6209aa34713e1417f4a2e24e5ca6 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.

GitOrigin-RevId: 369b2a783d90a501c7234ecfdd7c863bdb03ce3c
1 file changed