LibGfx+WindowServer: Remove set_size_around() from Rect and Window

Superceded by to_floating_cursor_position() as a more accurate way
to reposition windows on untile. Effectively made set_size_around()
dead code, so the remnants can be removed.

Userland/Libraries/LibGfx/Rect.h

@@ -107,8 +107,6 @@ public:
         m_size = size;
     }
 
-    void set_size_around(Size<T> const&, Point<T> const& fixed_point);
-
     void set_size(T width, T height)
     {
         m_size.set_width(width);