remove Direct2D renderer

Removes the old slow Direct2D renderer which has become obselete with
the new Direct3D11 renderer.  Note that Direct2D is built on top of
Direct3D so there's really no reason to keep it around.

src/win32/gui.zig

@@ -18,7 +18,7 @@ const RGB = @import("color").RGB;
 const input = @import("input");
 const windowmsg = @import("windowmsg.zig");
 
-const render = if (build_options.d2d) @import("d2d.zig") else @import("d3d11.zig");
+const render = @import("d3d11.zig");
 
 const FontFace = @import("FontFace.zig");
 const XY = @import("xy.zig").XY;