authPrompt: hide/stop spinner after verfiication completes

When the user successfully types their password, we should hide
the spinner from the button well right away, so it doesn't
consume resources until reset (which may happen significantly later
if the user is vt switched away)

https://bugzilla.gnome.org/show_bug.cgi?id=753891

js/gdm/authPrompt.js

@@ -258,6 +258,7 @@ const AuthPrompt = new Lang.Class({
     },
 
     _onVerificationComplete: function() {
+        this.setActorInDefaultButtonWell(null);
         this.verificationStatus = AuthPromptStatus.VERIFICATION_SUCCEEDED;
 	this.cancelButton.reactive = false;
     },