xdiff: remove unused functions

xdiff: remove unused functions

The functions xdl_cha_first(), xdl_cha_next() and xdl_atol() are not used
by us.  While removing them increases the difference to the upstream
version of libxdiff, it only adds a bit to the more than 600 differing
lines in xutils.c (mmfile_t management was simplified significantly when
the library was imported initially).  Besides, if upstream modifies these
functions in the future, we won't need to think about importing those
changes, so in that sense it makes tracking modifications easier.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>