Bug 1313358 - Clean up preference observer in ContentSearch. r=mak

Differential Revision: https://phabricator.services.mozilla.com/D102229

browser/actors/ContentSearchParent.jsm

@@ -147,6 +147,7 @@ let ContentSearch = {
       return this._destroyedPromise;
     }
 
+    Services.prefs.removeObserver("browser.search.hiddenOneOffs", this);
     Services.obs.removeObserver(this, "browser-search-engine-modified");
     Services.obs.removeObserver(this, "browser-search-service");
     Services.obs.removeObserver(this, "shutdown-leaks-before-check");