Bug 1566457 - Remove unused devtools-cleanup observer message. r=jdescottes

Differential Revision: https://phabricator.services.mozilla.com/D38282

--HG--
extra : moz-landing-system : lando

devtools/shared/Loader.jsm

@@ -93,13 +93,10 @@ var gNextLoaderID = 0;
  */
 this.DevToolsLoader = function DevToolsLoader() {
   this.require = this.require.bind(this);
-
-  Services.obs.addObserver(this, "devtools-unload");
 };
 
 DevToolsLoader.prototype = {
   destroy: function(reason = "shutdown") {
-    Services.obs.removeObserver(this, "devtools-unload");
 
     if (this._provider) {
       this._provider.unload(reason);
@@ -234,19 +231,6 @@ DevToolsLoader.prototype = {
     this.setProvider(new BuiltinProvider());
   },
 
-  /**
-   * Handles "devtools-unload" event
-   *
-   * @param String data
-   *    reason passed to modules when unloaded
-   */
-  observe: function(subject, topic, data) {
-    if (topic != "devtools-unload") {
-      return;
-    }
-    this.destroy(data);
-  },
-
   /**
    * Sets whether the compartments loaded by this instance should be invisible
    * to the debugger.  Invisibility is needed for loaders that support debugging