dashboards: revert adding api for retrieving uid by slug

Since we're already have possibility to get a dashboard by slug
it makes little sense to have a separate endpoint in api for
retrieving uid by slug.
#7883

pkg/models/dashboards.go

@@ -231,9 +231,3 @@ type GetDashboardSlugByIdQuery struct {
 	Id     int64
 	Result string
 }
-
-type GetDashboardUidBySlugQuery struct {
-	OrgId  int64
-	Slug   string
-	Result string
-}