Alerting: Remove legacy alerting (#83671)

Removes legacy alerting, so long and thanks for all the fish! 🐟

---------

Co-authored-by: Matthew Jacobson <matthew.jacobson@grafana.com>
Co-authored-by: Sonia Aguilar <soniaAguilarPeiron@users.noreply.github.com>
Co-authored-by: Armand Grillet <armandgrillet@users.noreply.github.com>
Co-authored-by: William Wernert <rwwiv@users.noreply.github.com>
Co-authored-by: Yuri Tseretyan <yuriy.tseretyan@grafana.com>

conf/provisioning/notifiers/sample.yaml

@@ -1,25 +0,0 @@
-# # config file version
-apiVersion: 1
-
-# notifiers:
-#   - name: default-slack-temp
-#     type: slack
-#     org_name: Main Org.
-#     is_default: true
-#     uid: notifier1
-#     settings:
-#       recipient: "XXX"
-#       token: "xoxb"
-#       uploadImage: true
-#       url: https://slack.com
-#   - name: default-email
-#     type: email
-#     org_id: 1
-#     uid: notifier2
-#     is_default: false  
-#     settings:
-#       addresses: example11111@example.com
-# delete_notifiers:
-#   - name: default-slack-temp
-#     org_name: Main Org.
-#     uid: notifier1