Revert "Work around missing PTHREAD_RWLOCK_INITIALIZER in NaCl newlib."

This reverts commit 68de407b5fc746c0fa82706dd897288d01c32930. The NaCl fix has
rolled into Chromium.

Change-Id: I9fd6a6ae727c95fa89b8ce27e301f2a748d0acc9
Reviewed-on: https://boringssl-review.googlesource.com/4651
Reviewed-by: Adam Langley <agl@google.com>