[OpenMP] Remove -Wno-enum-constexpr-conversion (#81318)

This effectively reverts commit
9ff0cc7e

. For some reason "git revert"
lead to "no changes" after fixing conflicts, so a clean revert was not
possible.

The original issue (#57022) is no longer reproducible even with this
patch, so we can remove the suppression.

This is in line with our goal to make -Wenum-constexpr-conversion a
non-downgradeable error, see #59036.

Co-authored-by: Carlos Gálvez <carlos.galvez@zenseact.com>