places-menu: Update fake mount operation source

gnome-shell now checks for an associated drive to automatically
cancel the operation on disconnect, so fake the corresponding
method. While at it, drop the `get_icon()` method that hasn't
been used for quite a while now.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/508
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/361>
(cherry picked from commit 8957f488dc)

extensions/places-menu/placeDisplay.js

@@ -61,8 +61,8 @@ class PlaceInfo extends EventEmitter {
                 return;
             }
 
-            let source = {
-                get_icon: () => this.icon,
+            const source = {
+                get_drive: () => null,
             };
             let op = new ShellMountOperation.ShellMountOperation(source);
             try {