pthread.h: drop nullability attributes.

These have been found to be practically useless. We were actually
following the Android bionic library and had some interest in replicating
the same warnings and behaviour but Android has since removed them.

We are still keeping some uses of nullability attributes in other headers,
somewhat in line with Apple's libc.

MFC after: 1 week
Hinted by: bionic (git 3f66e74b903905e763e104396aff52a81718cfde)