theme: Remove unnecessary %bubble_panel extensions

Since these dialogs have .modal-dialog class, there is no need to extend
%bubble_panel.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/931

data/theme/gnome-shell-sass/widgets/_network-dialog.scss

@@ -1,6 +1,5 @@
 /* Network Dialogs */
 .nm-dialog {
-  @extend %bubble_panel;
   max-height: 34em;
   min-height: 31em;
   min-width: 32em;