Revert "backport.yml: try to fix permissions"

This reverts commit 150d098.

.github/workflows/backport.yml

@@ -5,17 +5,12 @@ on:
       - closed
       - labeled
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   backport:
     runs-on: ubuntu-latest
     name: Backport
-
-    permissions:
-      pull-requests: write
-
     steps:
       - name: Backport Bot
         id: backport