hw/xwin: Remove nounicodeclipboard option

Always use CF_UNICODETEXT clipboard format.  Windows will automatically
down-convert to CF_TEXT for clients which request that.

This is subtly different in one way: if CF_TEXT is requested, we now
post CF_UNICODETEXT and it is converted to CF_TEXT *in the locale of the
requesting process*.  Previously, we would convert to CF_TEXT *in our
locale* and post that.

It looks like the code in the !X_HAVE_UTF8_STRING case didn't actually
work correctly, but fortunately that has never been true...