hppa: Remove unrequired nptl headers

Now that both pthread_mutex_t and pthread_rwlock_t static initializer
are parametrized in their own headers HPPA pthread.h is identical to
generic nptl one.

Checked on hppa-linux-gnu.

Change-Id: I236cfceb5656cfcce42c9e367a4f6803e2abd88b