Revert ".github: Add repo lockdown GitHub workflow and update .gitignore"

This reverts commit c531adbf08.

.github/workflows/repo-lockdown.yml

@@ -1,26 +0,0 @@
-name: 'Repo Lockdown'
-
-on:
-  pull_request_target:
-    types: opened
-
-permissions:
-  pull-requests: write
-
-jobs:
-  action:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: dessant/repo-lockdown@v4
-        with:
-          pr-comment: |
-            We have mailing list based patch review so it would be great if you can send these patchs to OpenSBI mailing list.
-
-            You need to join OpenSBI mailing list using following link
-            http://lists.infradead.org/mailman/listinfo/opensbi
-
-            Make sure you use "git send-email" to send the patches.
-
-            Thanks for your contribution to OpenSBI project.
-          lock-pr: true
-          close-pr: true