Revert "Workaround iOS UI input crash" (#585)

* Revert "Workaround iOS UI input crash"

This reverts commit 7df70e0.

# Conflicts:
#	workers/unity/Assets/Playground/Resources/Prefabs/Common/MobileConnectionSplashScreen.prefab
#	workers/unity/Assets/Playground/Scenes/iOSClientScene.unity
#	workers/unity/Assets/Playground/Scripts/UI/ConnectionScreenController.cs
#	workers/unity/Assets/Playground/Scripts/UI/ResizedGui.cs

* Adding back EventSystem to AndroidClientScene + ordering

* Reset errorMessage to empty string before connection

* Addressing PR comments

Author: gkassabli