Axe mouse cursor config option

The config option has been removed. I'm going to implement something
that automatically shows and hides the mouse cursor whenever
appropriate, which is better than a config option.

desktop_version/src/Graphics.cpp

@@ -125,7 +125,6 @@ void Graphics::init(void)
     warprect = SDL_Rect();
 
     translucentroomname = false;
-    showmousecursor = true;
 
     alpha = 1.0f;