[utils] Delete the 'check-coverage-regressions' script

In practice, it's way too noisy.

It's also a maintenance burden, since we apparently can't add tests for
it without breaking some Windows setups (see: D22692).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281871 91177308-0d34-0410-b5e6-96231b3b80d8