Project: delete the .jobsets jobset if project is no longer declarative

"No longer declarative" as defined by the "Edit project" page is an empty spec
file.

src/lib/Hydra/Controller/Project.pm

@@ -169,6 +169,8 @@ sub updateProject {
             , emailoverride => ""
             , triggertime => time
             });
+    } else {
+        $project->jobsets->search({ name => ".jobsets" })->delete;
     }
 }