[CMAKE] Fix scope issue in the remove_<lang>_compiler_flags() functions

Summary:
The CMAKE_<LANG>_FLAGS where not propagated to the parent scope.
The configuration specific variables were correctly scoped if a
configuration is defined, which is our default, but invalid if unset.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4584