screenshield: Remove box-shadow

This causes very low performance in some situations (like multiscreen). Proper
fixes are too invasive at this point (3.8.1) so lets just remove the shadow
for now and add it back later once he have fixed it.

https://bugzilla.gnome.org/show_bug.cgi?id=697274

data/theme/gnome-shell.css

@@ -2445,7 +2445,6 @@ StScrollBar StButton#vhandle:active {
 
 .screen-shield-background {
     background: black;
-    box-shadow: 0px 4px 8px rgba(0,0,0,0.9);
 }
 
 #lockDialogGroup {