magnifier: Unset variable holding GSource ID

Otherwise we try to remove the no longer existing GSource
later on.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1902>

js/ui/magnifier.js

@@ -1166,6 +1166,8 @@ var ZoomRegion = class ZoomRegion {
                 this._yDelayed = null;
             }
 
+            this._scrollContentsTimerId = 0;
+
             return GLib.SOURCE_REMOVE;
         });