[libc++abi] Remove unistd.h include

This was originally there for the _POSIX_THREADS define, to detect the
presence of pthreads. That went away with the externalized threading
support, so the include can go away too.

config.h is now completely empty. A follow-up commit will remove it
entirely.

llvm-svn: 299087

libcxxabi/src/config.h

@@ -14,6 +14,4 @@
 #ifndef LIBCXXABI_CONFIG_H
 #define LIBCXXABI_CONFIG_H
 
-#include <unistd.h>
-
 #endif // LIBCXXABI_CONFIG_H