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/extension.js

@@ -1,7 +1,6 @@
-import {ExtensionBase, GettextWrapper, setExtensionManager} from './sharedInternals.js';
+import {ExtensionBase, GettextWrapper} from './sharedInternals.js';
 
 const {extensionManager} = imports.ui.main;
-setExtensionManager(extensionManager);
 
 export class Extension extends ExtensionBase {
     static lookupByUUID(uuid) {