ci: remove redundant push trigger for main branch (#7)

Branch protections ensure commits to main can only be added via PR
(with bypass rules for release pipeline). Running CI on both the PR
and again after merge is redundant.

Author: drichar

.github/workflows/ci.yml

@@ -2,9 +2,6 @@ name: CI
 
 on:
   pull_request:
-  push:
-    branches:
-      - main
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}