ARCMT/GC-check-warn-nsalloc.m: don't provide a diag group

This diagnostic is now controlled solely by -no-ns-alloc-error thus matching
the original intended behaviour.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209184 91177308-0d34-0410-b5e6-96231b3b80d8