Revert "Skip build if "skip build" label is set"

This reverts commit 4db257d3.

It does not work on the manifest, because we do not run merge request
pipelines there, so CI_MERGE_REQUEST_LABELS is not set at all.

manifest-pipeline.yml

@@ -31,9 +31,6 @@ workflow:
 
 .full_build_pipeline:
   rules:
-    # Do not run build if the "skip build" label is set on the merge request
-    - if: $CI_MERGE_REQUEST_LABELS =~ /skip build/
-      when: never
     # Run the full build pipeline on non-master branches (i.e. in the merge request)
     # or if explicitly triggered by the API or the web button.
     - if: $CI_COMMIT_REF_NAME != $MASTER_BRANCH