Don't sync README to Docker Hub anymore

Starting with Icinga 2.16, the container builds are done from within that repo.
This also applies to the documentation that is now synced to Docker Hub from
there. Thus, this repo should no longer attempt to update the image description
on Docker Hub.

References:
- https://github.com/Icinga/icinga2/blob/master/.github/workflows/container-image.yml
- https://github.com/Icinga/icinga2/blob/master/.github/workflows/container-image.yml
- 30473c8975/.github/workflows/container-image.yml (L192-L228)

.github/workflows/sync-readme.yml

@@ -1,23 +0,0 @@
-name: Sync README.md to Docker Hub
-
-on:
-  push:
-    branches:
-      - master
-
-jobs:
-  sync:
-    name: Sync README
-    runs-on: ubuntu-latest
-
-    steps:
-      - name: Checkout HEAD
-        uses: actions/checkout@v1
-
-      - name: Sync README
-        uses: ms-jpq/sync-dockerhub-readme@e2991ea1ba48832e73555cdbd5b82f5a2e91ee9b # v1
-        with:
-          username: icingaadmin
-          password: '${{ secrets.DOCKER_HUB_PERSONAL_TOKEN }}'
-          repository: icinga/icinga2
-          readme: ./README.md