argp: Remove old includes in !_LIBC case

The headers mempcpy.h, strcase.h, strchrnul.h, and strndup.h are
included if not building argp for glibc. Commit
c5af724c added them in 2003 for gnulib,
but gnulib's current master patches them out:
https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/argp-namefrob.h;h=9c82ac79c215540f986c3e04398edba3ba1b7234;hb=HEAD