Revert "Send automated PR upon new stack release"

This reverts commit 730ffa8de5.

Lately, Elastic's release pipeline has been tagging releases from the
older 6.x series as "latest" in place of releases from the current 7.x
series. This results in weekly PRs from GitHub Actions for _downgrading_
our stack from 7.x to 6.x.

In order to avoid receiving those PRs, we disable the update workflow
entirely until the Elastic 6.x release series is marked as EOL.

Ref. elastic/elasticsearch#75436
Closes #611

.github/workflows/update.yml

@@ -1,45 +0,0 @@
-name: Update Elastic release
-
-on:
-  schedule:
-    - cron: '0 0 * * 0'  # At 00:00 every Sunday
-
-jobs:
-
-  check-and-update:
-    name: Check and update Elastic release
-    runs-on: ubuntu-latest
-
-    steps:
-      - uses: actions/checkout@v2
-
-      - name: Get latest release version
-        uses: octokit/request-action@v2.x
-        id: get_latest_release
-        with:
-          route: GET /repos/:repository/releases/latest
-          repository:  elastic/elasticsearch
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-
-      - name: Update stack version
-        run: |
-          source .env
-          cur_ver="$ELK_VERSION"
-          new_ver=${{ fromJson(steps.get_latest_release.outputs.data).tag_name }}
-
-          # Escape dot characters so sed interprets them as literal dots
-          cur_ver="$(echo $cur_ver | sed 's/\./\\./g')"
-          # Trim leading 'v' in semantic version
-          new_ver="${new_ver:1}"
-
-          for f in .env docker-stack.yml README.md; do
-            sed -i "s/${cur_ver}/${new_ver}/g" "$f"
-          done
-
-      - name: Send pull request to update to new version
-        uses: peter-evans/create-pull-request@v3
-        with:
-          commit-message: Update to ${{ fromJson(steps.get_latest_release.outputs.data).tag_name }}
-          title: Update to ${{ fromJson(steps.get_latest_release.outputs.data).tag_name }}
-          delete-branch: true