[TSan][libdispatch] Delete old tests

In a previous commit, I re-enabled the ported variants of these 2 tests:
tsan/Darwin/gcd-data.mm -> tsan/libdispatch/data.c
tsan/Darwin/gcd-source-serial.mm -> tsan/libdispatch/source-serial.c

So now we can delete the Darwin-only version.

llvm-svn: 358235