extensionSystem: Unset stylesheet file reference when unloaded

We must remove the GFile reference from the representing object when an
extension has been unloaded as this won't be used anymore later (e.g. as cached
ref).

js/ui/extensionSystem.js

@@ -76,6 +76,7 @@ function disableExtension(uuid) {
     if (extension.stylesheet) {
         let theme = St.ThemeContext.get_for_stage(global.stage).get_theme();
         theme.unload_stylesheet(extension.stylesheet);
+        delete extension.stylesheet;
     }
 
     try {