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>

docs/sources/developers/http_api/admin.md

@@ -615,8 +615,6 @@ Content-Type: application/json
 
 `POST /api/admin/provisioning/plugins/reload`
 
-`POST /api/admin/provisioning/notifications/reload`
-
 `POST /api/admin/provisioning/access-control/reload`
 
 `POST /api/admin/provisioning/alerting/reload`
@@ -637,7 +635,6 @@ See note in the [introduction]({{< ref "#admin-api" >}}) for an explanation.
 | provisioning:reload | provisioners:dashboards    | dashboards       |
 | provisioning:reload | provisioners:datasources   | datasources      |
 | provisioning:reload | provisioners:plugins       | plugins          |
-| provisioning:reload | provisioners:notifications | notifications    |
 | provisioning:reload | provisioners:alerting      | alerting         |
 
 **Example Request**: