[CMAKE] Split remove_compiler_flags into C and CXX variants

Summary: Keep a remove_compiler_flags macro that does both.

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja .. -DCMAKE_BUILD_TYPE=RelWithDebInfo
ninja -v

Check the -DNDEBUG does not appear on the compiler command lines.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2655