IM status menu: adjust the combo popup background

The transparent background for available/unavailable IM status menu
makes the text difficult to read.

Simplifies that by using the default combo-popup background instead of a
custom one.

https://bugzilla.gnome.org/show_bug.cgi?id=658091

data/theme/gnome-shell.css

@@ -551,7 +551,6 @@ StScrollBar StButton#vhandle:active {
 }
 
 .status-chooser-combo.popup-combo-menu {
-    background-color: rgba(0,0,0,0.7);
     padding: .4em 0em;
     border-radius: 4px;
     border: 1px solid #5f5f5f;