chore(NA): remove debug step on auto approve backport action (elastic…

…#160826)

I believe I found out what was wrong with the previous implementation of
the action.
We were trying to check for the existence of the backport label when the
PR is created which is never there (as it gets added afterwards).

This PR is replacing the checking of the label by the assert that the
branch name on a backport starts with `backport` and the creator of the
PR is kibanamachine.

.github/workflows/auto-approve-backports.yml

@@ -12,8 +12,8 @@ jobs:
     name: Auto-approve backport
     runs-on: ubuntu-latest
     if: |
-      contains(github.event.pull_request_target.labels.*.name, 'backport') &&
-      github.event.pull_request_target.user.login == 'kibanamachine'
+      startsWith(github.event.pull_request.head.ref, 'backport') &&
+      github.event.pull_request.user.login == 'kibanamachine'
     permissions:
       pull-requests: write
     steps: