AM_PROG_MKDIR_P: remove as obsolete

AM_PROG_MKDIR_P: remove as obsolete

Support for the obsolescent 'AM_PROG_MKDIR_P' m4 macro (and its
output variable '$(mkdir_p)') has in the documentation and with
runtime warning since Automake 1.12.1.  It's now time to remove it.

* automake.in (scan_autoconf_traces): Remove special handling of
'AM_PROG_MKDIR_P'.
* m4/mkdirp.m4: Delete.
* Makefile.am (dist_automake_ac_DATA): Remove it.
* t/mkdirp-deprecation.sh: Delete.
* t/list-of-tests.mk: Remove it.
* doc/automake.texi: Remove references to the 'AM_MKDIR_P' m4 macro
and the '$(mkdir_p)' output variable.
* t/gettext-macros.sh: Ensure the gettext-requiring tests will
still see the now-removed 'AM_PROG_MKDIR_P' macro as an alias for
the 'AC_PROG_MKDIR_P' macro, since even recent versions of gettext
uses 'AM_PROG_MKDIR_P' in their '.m4' files.
* NEWS: Update.

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