ci: removed merge-variants workflow since GHA won't trigger branch builds

.github/workflows/merge-variants.yml

@@ -1,20 +0,0 @@
-name: Merge image variants
-on:
-  push:
-    branches:
-      - master
-jobs:
-  merge:
-    if: github.repository == 'itzg/docker-minecraft-server'
-    runs-on: ubuntu-latest
-    timeout-minutes: 5
-    steps:
-      - uses: actions/checkout@v2.3.4
-        with:
-          # fetch all history for merging operations
-          fetch-depth: 0
-      - name: Run docker-versions-create
-        run: |
-          git config --add user.name "${GITHUB_ACTOR:-Github Actions}"
-          git config --add user.email "actions@github.com"
-          ./docker-versions-create.sh -b