ptx: remove use of diacrit module

ptx: remove use of diacrit module

The diacrit module is obsolete, and ptx’s use of it is obsolete
too; it assumes an 8-bit locale (not that common these days) and
that TeX cannot process the 8-bit characters (nowadays, it can).
* NEWS, doc/coreutils.texi (Charset selection in ptx): Document this.
* bootstrap.conf (gnulib_modules): Remove diacrit.
* src/ptx.c: Do not include diacrit.h.
(print_field, fix_output_parameters): Remove obsolete support
for 8-bit diacritics.