docking: Use St.IconTheme directly, without wrappers

The old code was using Gtk as fallback to support old versions but this
is not the case anymore.

docking.js

@@ -1653,7 +1653,7 @@ export class DockManager {
         this._appSwitcherSettings = new Gio.Settings({schema_id: 'org.gnome.shell.app-switcher'});
         this._mapSettingsValues();
 
-        this._iconTheme = new Utils.IconTheme();
+        this._iconTheme = new St.IconTheme();
 
         this._desktopIconsUsableArea = new DesktopIconsIntegration.DesktopIconsUsableAreaClass();
         this._oldDash = Main.overview.isDummy ? null : Main.overview.dash;
@@ -1740,7 +1740,7 @@ export class DockManager {
     }
 
     get iconTheme() {
-        return this._iconTheme.iconTheme;
+        return this._iconTheme;
     }
 
     get fm1Client() {
@@ -2647,7 +2647,7 @@ export class DockManager {
         Locations.unWrapFileManagerApp();
         this._removables?.destroy();
         this._removables = null;
-        this._iconTheme.destroy();
+        this._iconTheme = null;
         this._remoteModel?.destroy();
         this._settings = null;
         this._appSwitcherSettings = null;

utils.js

@@ -3,7 +3,6 @@ import {
     GLib,
     Gio,
     GObject,
-    Gtk,
     Meta,
     Shell,
     St,
@@ -490,33 +489,6 @@ export function splitHandler(handler) {
     });
 }
 
-export class IconTheme {
-    constructor() {
-        if (St.IconTheme) {
-            this._iconTheme = new St.IconTheme();
-            return;
-        }
-
-        const settings = St.Settings.get();
-        this._iconTheme = new Gtk.IconTheme();
-        this._iconTheme.set_custom_theme(settings.gtkIconTheme);
-        this._changesId = settings.connect('notify::gtk-icon-theme', () => {
-            this._iconTheme.set_custom_theme(settings.gtkIconTheme);
-        });
-    }
-
-    get iconTheme() {
-        return this._iconTheme;
-    }
-
-    destroy() {
-        if (this._changesId)
-            St.Settings.get().disconnect(this._changesId);
-
-        this._iconTheme = null;
-    }
-}
-
 /**
  * Construct a map of gtk application window object paths to MetaWindows.
  */