popupMenu: Remove combo boxes and child menus

They're no longer used with the removal of the avatar widget.

https://bugzilla.gnome.org/show_bug.cgi?id=704368

data/theme/gnome-shell.css

@@ -189,13 +189,6 @@ StScrollBar StButton#vhandle:active {
     border-width: 0px;
 }
 
-.popup-combo-menu {
-    background-color: rgba(0,0,0,0.9);
-    padding: 1em 0em;
-    border: 1px solid #5f5f5f;
-    border-radius: 9px;
-}
-
 /* The remaining popup-menu sizing is all done in ems, so that if you
  * override .popup-menu.font-size, everything else will scale with it.
  */
@@ -219,10 +212,6 @@ StScrollBar StButton#vhandle:active {
 .popup-image-menu-item {
 }
 
-.popup-combobox-item {
-    spacing: 1em;
-}
-
 .popup-separator-menu-item {
     -gradient-height: 1px;
     -gradient-start: rgba(255,255,255,0.0);