build: Remove the Auto-merge automation.

This automation is not documented and has not been used in over seven
months. Remove references to it before we remove it from the
organization.

See https://github.com/openedx/axim-engineering/issues/898 for more
details.

.github/workflows/pr-automerge-open-release.yml

@@ -1,24 +0,0 @@
-# Enable automerging for named release branches.
-# See the reusable workflow for details:
-# https://github.com/openedx/.github/.github/workflows/pr-automerge-open-release.yml
-
-name: Automerge BTR open-release PRs
-
-on:
-  issue_comment:
-    branches:
-      - open-release/*
-    types:
-      - created
-      - edited
-  pull_request_target:
-    branches:
-      - open-release/*
-    types:
-      - opened
-      - edited
-      - ready_for_review
-
-jobs:
-  automerge:
-    uses: openedx/.github/.github/workflows/pr-automerge-open-release.yml@master