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>

pkg/api/org_invite.go

@@ -350,6 +350,9 @@ func (hs *HTTPServer) applyUserInvite(ctx context.Context, usr *user.User, invit
 	return true, nil
 }
 
+// swagger:response SMTPNotEnabledError
+type SMTPNotEnabledError PreconditionFailedError
+
 // swagger:parameters addOrgInvite
 type AddInviteParams struct {
 	// in:body