Remove not used integration with Open Community Build (scala#16751)

This change removes integration with Open Community Build, triggering
builds after publishing a new nightly version. This feature was not
really used previously - nightly builds we're testing only 100 - 200
projects, and we relied on weekly builds (with over 1k projects) anyway.

This change makes project secret `BUILD_TOKEN` obsolete.

.github/workflows/ci.yaml

@@ -585,45 +585,6 @@ jobs:
           external_repository: lampepfl/dotty-website
           publish_branch: gh-pages
 
-  nightly_unmanaged_community_build:
-    # Self-hosted runner is used only for getting current build version
-    runs-on: [self-hosted, Linux]
-    container:
-      image: lampepfl/dotty:2021-03-22
-      options: --cpu-shares 4096
-      volumes:
-        - ${{ github.workspace }}/../../cache/sbt:/root/.sbt
-        - ${{ github.workspace }}/../../cache/ivy:/root/.ivy2/cache
-        - ${{ github.workspace }}/../../cache/general:/root/.cache
-    needs: [publish_nightly]
-    if: "(github.event_name == 'schedule' || github.event_name == 'workflow_dispatch') && github.repository == 'lampepfl/dotty'"
-    env:
-      NIGHTLYBUILD: yes
-    steps:
-      - name: Reset existing repo
-        run: git -c "http.https://github.com/.extraheader=" fetch --recurse-submodules=no "https://github.com/lampepfl/dotty" && git reset --hard FETCH_HEAD || true
-
-      - name: Checkout cleanup script
-        uses: actions/checkout@v3
-
-      - name: Cleanup
-        run: .github/workflows/cleanup.sh
-
-      - name: Git Checkout
-        uses: actions/checkout@v3
-
-      - name: Add SBT proxy repositories
-        run: cp -vf .github/workflows/repositories /root/.sbt/ ; true
-
-      - name: Get version string for this build
-        run: |
-          ver=$(./project/scripts/sbt "print scala3-compiler-bootstrapped/version" | tail -n1)
-          echo "This build version: $ver"
-          echo "THISBUILD_VERSION=$ver" >> $GITHUB_ENV
-      # Steps above are copy-pasted from publish_nightly, needed only to resolve THISBUILD_VERSION
-      - name: Trigger unmanaged community build
-        run: .github/workflows/scripts/triggerUnmanagedCommunityBuild.sh "${{ secrets.BUILD_TOKEN }}" "$THISBUILD_VERSION"
-
   publish_release:
     permissions:
       contents: write  # for actions/create-release to create a release