Simplify x-dnd.el due to bignums

Simplify x-dnd.el due to bignums

* lisp/x-dnd.el (x-dnd-get-drop-x-y, x-dnd-version-from-flags)
(x-dnd-more-than-3-from-flags, x-dnd-get-motif-value)
(x-dnd-motif-value-to-list): Do not worry about pairs of
16-bit numbers, as the C code no longer generates them;
it generates bignums now, when needed on 32-bit platforms.