Remove unused GUIUtil functions, unused GUI includes and some tails in GUIUtil.
HasSetting from rP22693 is actually redundant with IGUIObject::SettingExists.
GetSettingPointer from rP1518 is superseded by GetSetting reference following rP22693 and rP22765.
Deregister copying SetSetting variants for CStr and CStrW following copy removal in rP22765.
The default template <typename T=int> from rP31 can be removed following FallBackSprite/FallBackColor removal in rP22689 and RecurseObject unification in rP22744.
Delete all unused GUI includes, refs D1478.
Remove GUIUtil friend class following something.
Differential Revision: https://code.wildfiregames.com/D2225
Tested on: gcc 9.1.0, Jenkins