Don't look for pthread_mutexattr_setrobust_np

Don't look for pthread_mutexattr_setrobust_np

Apparently Red Hat's Fedora Core 5 defines it but doesn't declare it,
so we'd have to declare it before testing the address.  While it was
once useful for checking whether the pthread code had been loaded, I
think the other tests done now are more effective and this isn't
needed any more.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18012 dc483132-0cff-0310-8789-dd5450dbe970