Remove unneeded header file which does not exist on Darwin.
Disable test for darwin as pthread_setname_np is not implemented.
But setting and observing the threadname is what this test is all about.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14573 a5019735-40e9-0310-863c-91ae7b9d1cf9