[ASAN][DARWIN] Remove getpwnam(NULL) test for undefined behavior

Reverting a patch that was added to test for getpwnam(NULL) -- it was noted at the time the behavior might have been a bug, however the patch was added for binary compatibility. Because of the change in the expected behavior, we are reverting this commit, as the test added is no longer passing.

Update: Rather than reverting the original commit, updating this to only remove the unnecessary test.

Original Patch: https://reviews.llvm.org/D40052

rdar://98592334