Remove wrong wx/windowid.h include, should never be done directly

This #include was mistakenly added by e8b8b0288f (Make wxNewId() and
others return/take wxWindowID rather than int, 2019-12-18) and broke
compilation of the code including wx/utils.h as the first header,
because wx/windowid.h can't be included directly and must be only
included from wx/defs.h.

Fix this by just removing it, including any other header is enough to
pull this one in anyhow, via wx/defs.h.

See https://github.com/wxWidgets/wxWidgets/pull/1682

Closes https://github.com/wxWidgets/wxWidgets/pull/1815

include/wx/utils.h

@@ -15,7 +15,6 @@
 // headers
 // ----------------------------------------------------------------------------
 
-#include "wx/windowid.h"
 #include "wx/object.h"
 #include "wx/list.h"
 #include "wx/filefn.h"