css: Remove style transitions for the panel

This, together with the panel's transparent background in the screen
shield, has the unfortunate side-effect of showing the desktop
background in a brief flash while coming out of the screen shield.

https://bugzilla.gnome.org/show_bug.cgi?id=692966

data/theme/gnome-shell.css

@@ -429,7 +429,6 @@ StScrollBar StButton#vhandle:active {
     background-color: black;
     font-weight: bold;
     height: 1.86em;
-    transition-duration: 250ms;
 }
 
 #panel.lock-screen {