libgettextpo: Remove unused object files.
* gettext-tools/src/read-po-lex.h: Don't include xerror.h.
* gettext-tools/libgettextpo/Makefile.am (libgettextpo_la_AUXSOURCES): Remove
po-xerror.c and po-error.c.
* autogen.sh (GNULIB_MODULES_LIBGETTEXTPO): Remove xerror.
