Remove unnecessary Mac-specific IsShownOnScreen() implementation

It ended up returning the result of the base class method anyhow, just
after doing a lot of unnecessary work, so this commit shouldn't change
anything, but should significantly speed up this function.

We may want to explore the possibility of implementing this method
natively using GetPeer()->IsVisible() later, as it could be faster than
our own implementation, but we need to ensure that this always returns
the same result as before, which might not be the case when the TLW is
in the process of being shown or hidden, so don't do this for now.

Closes https://github.com/wxWidgets/wxWidgets/pull/1911

Closes #18645.

include/wx/osx/window.h

@@ -72,8 +72,6 @@ public:
         return OSXShowWithEffect(false, effect, timeout);
     }
 
-    virtual bool IsShownOnScreen() const wxOVERRIDE;
-
     virtual void SetFocus() wxOVERRIDE;
 
     virtual void WarpPointer( int x, int y ) wxOVERRIDE;