Remove signout flash message because it also makes no sense.

You get redirected after signuout to gitlab.com and see this message on login page which makes no sense.

config/locales/devise.en.yml

@@ -24,7 +24,7 @@ en:
       inactive: 'Your account was not activated yet.'
     sessions:
       signed_in: ''
-      signed_out: 'Signed out successfully.'
+      signed_out: ''
     users_sessions:
       user:
         signed_in: 'Signed in successfully.'