ci: assigner: only run on main branch

See if this fixes the issue where this workflow runs on released
branches.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

.github/workflows/assigner.yml

@@ -9,8 +9,6 @@ on:
     - ready_for_review
     branches:
     - main
-    - collab-*
-    - v*-branch
   issues:
     types:
     - labeled