Remove voxmedia action (#1782)

This PR removes the Slack notification from the `test-from-pypi` GitHub
Action. The voxmedia
[action](https://github.com/voxmedia/github-action-slack-notify-build)
we were using is no longer maintained.

.github/workflows/test-from-pypi.yml

@@ -115,31 +115,3 @@ jobs:
     - name: Test Traits package (Windows/macOS)
       run: cd testdir && python -m unittest discover -v traits
       if: matrix.os != 'ubuntu-latest'
-
-  notify-on-failure:
-    needs: [test-pypi-sdist, test-pypi-wheel]
-    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-pypi-sdist, test-pypi-wheel]
-    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 }}