window: Kill off the rest of colormaps

I removed the XInstallColormap / XUninstallColormap but forgot
to remove the fields in MetaWindow and the code to set them.

src/core/window-private.h

@@ -103,7 +103,6 @@ struct _MetaWindow
   MetaFrame *frame;
   int depth;
   Visual *xvisual;
-  Colormap colormap;
   char *desc; /* used in debug spew */
   char *title;