Remove redundant branches-ignore from image.yml trigger

Same fix as testing repo: branches-ignore under push was causing
spurious workflow runs on branch pushes. With only tags filter
present, branch pushes are already excluded.

Author: luigidematteis

.github/workflows/image.yml

@@ -22,8 +22,6 @@ on:
   push:
     tags:
     - '[0-9]*'
-    branches-ignore:
-    - '*'
 jobs:
   image:
     name: Build OpenServerless Operator Image