pthread.h: Remove PTHREAD_RWLOCK_INITIALIZER macro.

This reverts commit 17286dc70a.

As discussed on the mailing list. As it turns out, this was less
than half of an actual implementation of this macro, and there's
technically no way to implement it without introducing (theoretical)
race conditions, in the current design anyway.

headers/posix/pthread.h

@@ -80,8 +80,6 @@ extern "C" {
 	{ PTHREAD_MUTEX_RECURSIVE, 0, -42, -1, 0 }
 #define PTHREAD_COND_INITIALIZER	\
 	{ 0, -42, NULL, 0, 0 }
-#define PTHREAD_RWLOCK_INITIALIZER \
-	{ 0, 0, NULL }
 
 /* mutex functions */
 extern int pthread_mutex_destroy(pthread_mutex_t *mutex);