extensionBase: Stop injecting extensionManager

All extension lookups now happen through the dedicated
Extension/ExtensionPreferences classes, so the shared
code no longer has to access the extensionManager.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2838>

js/extensions/prefs.js

@@ -1,10 +1,8 @@
 import GObject from 'gi://GObject';
 
-import {ExtensionBase, GettextWrapper, setExtensionManager} from './sharedInternals.js';
+import {ExtensionBase, GettextWrapper} from './sharedInternals.js';
 import {extensionManager} from '../extensionsService.js';
 
-setExtensionManager(extensionManager);
-
 export class ExtensionPreferences extends ExtensionBase {
     static lookupByUUID(uuid) {
         return extensionManager.lookup(uuid)?.stateObj ?? null;