Remove lcore-migration branch trigger from CI

The lcore-migration branch is now main in the new repo,
so the separate trigger is no longer needed.

Author: omkarjoshi0304

.github/workflows/build-and-push.yaml

@@ -4,7 +4,6 @@ on:
   push:
     branches:
       - main
-      - lcore-migration
   workflow_dispatch: {}
 
 env: