screenShield: Stop using an offscreen buffer for the arrow actor

This isn't a performance critical actor and the NVIDIA driver discards
offscreen buffers in some cases which would require us to go through
extra hoops to handle here which isn't worth it.

https://bugzilla.gnome.org/show_bug.cgi?id=739178

js/ui/screenShield.js

@@ -349,7 +349,6 @@ const Arrow = new Lang.Class({
     _init: function(params) {
         this.parent(params);
         this.x_fill = this.y_fill = true;
-        this.set_offscreen_redirect(Clutter.OffscreenRedirect.ALWAYS);
 
         this._drawingArea = new St.DrawingArea();
         this._drawingArea.connect('repaint', Lang.bind(this, this._drawArrow));