[TOOLING] Delete notify_weekly.yaml file (#10705)

Removing this uneeded workflow. This was disabled in the job runs
already. The file still needs to be deleted.

Can be replaced with "saved views" queries if any one on the team would
like to use them:

`is:issue is:open sort:comments-desc updated:>@today-1w org:certbot` and
`is:pr is:open updated:>@today-1w org:certbot`

.github/workflows/notify_weekly.yaml

@@ -1,29 +0,0 @@
-name: Weekly Github Update
-
-on:
-  schedule:
-    # Every week on Thursday @ 10:00
-    - cron: "0 10 * * 4"
-  workflow_dispatch:
-permissions: {}  # let's not use any permissions we don't need here
-jobs:
-  send-mattermost-message:
-    runs-on: ubuntu-latest
-
-    steps:
-    - name: Create Mattermost Message
-      run: |
-        DATE=$(date --date="7 days ago" +"%Y-%m-%d")
-        echo "ASSIGNED_PRS=https://github.com/pulls?q=is%3Apr+is%3Aopen+updated%3A%3E%3D${DATE}+assignee%3A*+user%3Acertbot" >> $GITHUB_ENV
-        echo "UPDATED_URL=https://github.com/issues?q=is%3Aissue+is%3Aopen+sort%3Acomments-desc+updated%3A%3E%3D${DATE}+user%3Acertbot" >> $GITHUB_ENV
-    # we pin this action to a version tested and audited by certbot's
-    # maintainers for extra security. the full hash is used as doing so is
-    # recommended by zizmor
-    - uses: mattermost/action-mattermost-notify@ae31bb6f9e26a54336e79696f108a2c91cf55b4e
-      with:
-        MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_WEBHOOK_URL }}
-        MATTERMOST_CHANNEL: private-certbot
-        TEXT: |
-          ## Updates In the Past Week
-          - Most commented in the last week: [link](${{ env.UPDATED_URL }})
-          - Updated (assigned) PRs in the last week: [link](${{ env.ASSIGNED_PRS }})