[flang] Fix release build flags.

Summary:
This patch removes the custom CMAKE_RELEASE_CXX_FLAGS variable.
This variable being set was having the effect of removing other important
Release flags, notably `-DNDEBUG`.

This patch may need to be accompanied by fixes for the macOS issues that
the removed comment mentions; I don't have a mac to test this on though so
hopefully a reviewer can help with that.

Reviewers: Andrzej, tskeith, sscalpone

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80794