panel: Use dedicated Spinner class

The startup/busy indication in the app menu was left out of commit
22e21ad7d1 because it doesn't use a hard-coded image, but as the
image in the CSS is actually the same used by the spinner class,
drop the "custom" styling and use the regular spinner.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/636

data/theme/gnome-shell-sass/_common.scss

@@ -1171,7 +1171,6 @@ StScrollBar {
   // a little unstructured mess:
 
   #appMenu {
-    spinner-image: url("resource:///org/gnome/shell/theme/process-working.svg");
     spacing: 4px;
 
     .label-shadow { color: transparent; }