screenShield: Remove blur and desaturation from lock screen

There's large performance issues with both the blur and desaturation
that make the screen shield hard to use on slower computers, and this
has always been a temporary stopgap until the user can pick a different
image for the lock screen.

https://bugzilla.gnome.org/show_bug.cgi?id=696322

js/ui/screenShield.js

@@ -547,9 +547,7 @@ const ScreenShield = new Lang.Class({
 
         let bgManager = new Background.BackgroundManager({ container: widget,
                                                            monitorIndex: monitorIndex,
-                                                           effects: Meta.BackgroundEffects.BLUR | Meta.BackgroundEffects.DESATURATE,
                                                            controlPosition: false });
-        bgManager.background.saturation = 0.6;
 
         this._bgManagers.push(bgManager);