Remove slack notification action (#350)

The Slack notification action that we were using is no longer supported,
and there isn't an obvious good replacement. This PR removes it.

.github/workflows/ets-from-source.yml

@@ -43,31 +43,3 @@ jobs:
         run: python etstool.py install --runtime=${{ matrix.runtime }} --source
       - name: Run tests
         run: python etstool.py test --runtime=${{ matrix.runtime }}
-
-  notify-on-failure:
-    needs: test-with-edm
-    if: failure()
-    runs-on: ubuntu-latest
-    steps:
-      - name: Notify Slack channel on failure
-        uses: voxmedia/github-action-slack-notify-build@v1
-        with:
-          channel_id: ${{ secrets.ETS_SLACK_CHANNEL_ID }}
-          status: FAILED
-          color: danger
-        env:
-          SLACK_BOT_TOKEN: ${{ secrets.SLACK_ACTION_SECRET }}
-
-  notify-on-success:
-    needs: test-with-edm
-    if: success()
-    runs-on: ubuntu-latest
-    steps:
-      - name: Notify Slack channel on success
-        uses: voxmedia/github-action-slack-notify-build@v1
-        with:
-          channel_id: ${{ secrets.ETS_BOTS_SLACK_CHANNEL_ID }}
-          status: SUCCESS
-          color: good
-        env:
-          SLACK_BOT_TOKEN: ${{ secrets.SLACK_ACTION_SECRET }}