[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