CI: Do not allow build job failures for Fedora 35

The CI build jobs targeting Fedora 35 pass (without compiler warnings).
Enforce this going forward.

.gitlab-ci.yml

@@ -82,7 +82,6 @@ fedora-35-clang:
     CONFIGURE_OPTS: "$CONFIGURE_MINI $CONFIGURE_FULL"
     MAKE_FLAGS: "CFLAGS=-Werror CXXFLAGS=-Werror"
   <<: *compile_definition
-  allow_failure: true
 
 alpine-3.15-musl:
   image: $REGISTRY_HUB:alpine-3.15-musl