ci: retire GitHub-Action Slack notifier for new issues and PRs (#9065)

Removes the rtCamp/action-slack-notify pings:

- Deletes .github/workflows/notify-slack.yml (issue-opened notifier).
- Drops the "Send Slack Notification for External PR" step from
  pr-labeler.yml; the labeler still applies pr/internal vs
  pr/external + stage/needs-triage labels as before.

Slack notifications for new aws-sam-cli issues and PRs are now sent by
the team's GitHub-App-backed notifier (aws-sam-tooling-bot), which
covers all of the team's owned repositories from a single backend
rather than per-repo workflows. Retiring the workflow-based path here
removes the duplicate posts and the long-lived SLACK_WEBHOOK_URL
secret dependency on this repo's behalf.

Author: roger-zhangg

.github/workflows/notify-slack.yml

@@ -20,7 +20,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Apply internal/external label
-        id: label_step
         uses: actions/github-script@v9
         with:
           github-token: ${{secrets.GITHUB_TOKEN}}
@@ -29,7 +28,7 @@ jobs:
               'aws-sam-cli-bot',
               'seshubaws', 'valerena',
               'Vandita2020', 'roger-zhangg',
-              'vicheey', 'bnusunny', 'tobixlea', 
+              'vicheey', 'bnusunny', 'tobixlea',
               'reedham-aws', 'licjun', 'dependabot[bot]'
             ];
             if (maintainers.includes(context.payload.sender.login)) {
@@ -39,22 +38,11 @@ jobs:
                 repo: context.repo.repo,
                 labels: ['pr/internal']
               });
-              core.setOutput("external", "false");
             } else {
               await github.rest.issues.addLabels({
                 issue_number: context.issue.number,
                 owner: context.repo.owner,
                 repo: context.repo.repo,
                 labels: ['pr/external', 'stage/needs-triage']
               });
-              core.setOutput("external", "true");
             }
-      - name: Send Slack Notification for External PR
-        if: steps.label_step.outputs.external == 'true'
-        uses: rtCamp/action-slack-notify@e31e87e03dd19038e411e38ae27cbad084a90661 # v2
-        env:
-          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK_URL }}
-          SLACK_TITLE: 'PR Created: ${{ github.event.pull_request.title }} by ${{ github.event.pull_request.user.login }}'
-          SLACK_FOOTER: ''
-          MSG_MINIMAL: true
-          SLACK_MESSAGE: '${{ github.event.pull_request.html_url }}'