Remove double trigger on merge_request event for coverage fuzzing

changelogs/unreleased/bugfix_coverage_fuzzing_merge_event.yml

+---
+title: coverage fuzzing bugfix - remove merge_request event trigger
+merge_request: 41906
+author:
+type: fixed

lib/gitlab/ci/templates/Security/Coverage-Fuzzing.gitlab-ci.yml

@@ -34,5 +34,5 @@ variables:
   rules:
     - if: $COVFUZZ_DISABLED
       when: never
-    - if: $GITLAB_FEATURES =~ /\bcoverage_fuzzing\b/
+    - if: $CI_COMMIT_BRANCH && $GITLAB_FEATURES =~ /\bcoverage_fuzzing\b/
     - if: $CI_RUNNER_EXECUTABLE_ARCH == "linux"