Remove push trigger to avoid duplicate CI runs (#300)

Having both push and pull_request triggers causes duplicate CI runs for
upstream branch PRs. The pull_request trigger alone covers both fork
and upstream PRs.

Author: mattzcarey

.github/workflows/branches.yml

@@ -1,7 +1,5 @@
 name: CI
 on:
-  push:
-    branches-ignore: ['main']
   pull_request:
     branches: ['main']