Cleanup XEditRes hack

Cleanup XEditRes hack

* configure.ac [USE_X_TOOLKIT]: Define X_TOOLKIT_EDITRES if
_XEditResCheckMessages is declared in X11/Xmu/Editres.h and may be
linked with -lXmu.  This should work with any non-ancient Xmu library.
* xfns.c (toplevel): Remove old cruft.
(x_window) [USE_X_TOOLKIT]: Use X_TOOLKIT_EDITRES.
* xterm.c (toplevel): Remove old cruft.
(handle_one_xevent): Use X_TOOLKIT_EDITRES.
* xterm.h (toplevel): Include X11/Xmu/Editres.h if X_TOOLKIT_EDITRES.