[msan] Remove cxa_atexit_race.cc

Summary:
The goal of the test to check that msan does not crash when code is racy on __cxa_atexit. Original crash was caused by race condition in the glibc. With
the msan patch the msan does not crashes however the race is still there and the test triggers it.

Because the test relies on triggering of undefined behavior results are not
very predictable and it may occasionally crashes or hangs.

I don't see how to reasonably improve the test, so I remove it.

Reviewers: eugenis, peter.smith

Subscribers: jfb, jdoerfert, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D58396

llvm-svn: 354377