network: Make the device section headers not bold

The new device section headers will be simple text strings
in the new designs.

This is a part of the new system status design, see
https://wiki.gnome.org/GnomeShell/Design/Guidelines/SystemStatus/
for design details.

https://bugzilla.gnome.org/show_bug.cgi?id=704670

data/theme/gnome-shell.css

@@ -234,11 +234,6 @@ StScrollBar StButton#vhandle:active {
     color: white;
 }
 
-.popup-subtitle-menu-item, .popup-subtitle-menu-item:insensitive {
-    font-weight: bold;
-    color: white;
-}
-
 .popup-menu-icon {
     icon-size: 1.09em;
 }