Remove unused attribute Graphics::tl

For some reason, this `tl` is a `point`? But the only other time the
name `tl` is used elsewhere in the code is a float on a `textboxclass`.
Regardless, this is unused.

desktop_version/src/Graphics.cpp

@@ -117,7 +117,6 @@ void Graphics::init()
     menubuffer = NULL;
     screenbuffer = NULL;
     tempBuffer = NULL;
-    tl = point();
     towerbuffer = NULL;
     towerbuffer_lerp = NULL;
     footerbuffer = NULL;