OpenAPI: Fix alerting DeleteMuteTiming errors (#91109)

The `GenericPublicError` is not what is actually returned by the API. Using `PublicError` describes the API correctly

pkg/services/ngalert/api/tooling/definitions/shared.go

@@ -20,10 +20,3 @@ type ForbiddenError struct {
 	// in: body
 	Body errutil.PublicError `json:"body"`
 }
-
-// swagger:model
-type GenericPublicError struct {
-	// The response message
-	// in: body
-	Body errutil.PublicError `json:"body"`
-}