ci: Revert "Cover compilation with Clang on Fedora in every merge request"

Revert commit cc1f631e38 (ci: Cover compilation with Clang on Fedora in
every merge request, 2026-01-13) and commit 5ccf55ffa7 (ci: Decouple
Fedora+Clang common job settings from external testing, 2026-01-13).
The motivation was to enable clang-specific features, but that turns
out to need a separate job anyway.