Fix: Don't use globalThis, since it won't get g.c. on window close

Don't use `globalThis`, since it won't get garbage collected on window
close.

See here for the relevant review:
https://extensions.gnome.org/review/38364
And for the relevant documentation:
https://gjs.guide/extensions/review-guidelines/review-guidelines.html#only-use-init-for-initialization

src/prefs.js

@@ -11,6 +11,4 @@ function buildPrefsWidget() {
 }
 
 function init() {
-    // Load the settings.
-    globalThis.settings = ExtensionUtils.getSettings();
 }