aclocal: remove the --acdir option

aclocal: remove the --acdir option

It not documented in the manual anymore, its use has already been
causing a deprecation warning, and its oncoming removal has already
been announced in the NEWS file.  So let's finally remove it.

* aclocal.in (parse_arguments): Don't recognize the '--acdir' option
anymore.
(handle_acdir_option): Remove subroutine.
* configure.ac ($ACLOCAL): Adjust.
($AUTOMAKE): Cosmetic changes, for consistency.
* tests/aclocal-acdir.test: Remove usages of the '--acdir' option.
* tests/aclocal-print-acdir.test: Likewise.
* NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>