Remove AC_HEADER_DIRENT

Remove AC_HEADER_DIRENT

POSIX.1-2001 defines 'struct dirent' in <dirent.h>.  It replaces
the old 'struct direct' found in BSDs.  All of the systems that I
checked (including FreeBSD, NetBSD, and OpenBSD), now provide
<dirent.h> with 'struct dirent', as mandated by POSIX.

Since autoconf first checks <dirent.h> and only if it's missing it
checks other header files, it's clear that it will always find
<dirent.h>, so let's simplify.

GNU autoconf documentation declares this macro as obsolescent, and
acknowledges that all current systems with directory libraries
have <dirent.h>:
<https://www.gnu.org/software/autoconf/manual/autoconf-2.70/html_node/Particular-Headers.html>

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>