Remove `tsan/Darwin/gcd-after-null.mm` test.

Summary:
This test invokes undocumented behaviour that could change in
the future. Given this, it's probably best to just remove the
test.

rdar://problem/42022283

Reviewers: kubamracek

Subscribers: llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D49269

llvm-svn: 336977