Remove weaker _POSIX_THREAD_PROCESS_SHARED test.

This branch (master, which will be o-mr1) already has a stronger test
than this one (which was added for o-dr, which doesn't have its own API
level).

Bug: http://b/62424597
Test: ran tests
Change-Id: Icfb7b50fa3c5b6d256d378add12c854a899a1c92