Avoid running mergeback workflow on PR

This was causing the workflow to run and create tags after merging
main into a branch.

Author: aeisenberg

.github/workflows/post-release-mergeback.yml

@@ -16,10 +16,6 @@ on:
     branches:
       - v1
 
-  pull_request:
-    paths:
-      - .github/workflows/post-release-mergeback.yml
-
 jobs:
   merge-back:
     runs-on: ubuntu-latest