Drop redundant call to AC_PROG_MKDIR_P

This reverts commit fc792d9edf4cb8b08cbad5bdf349a0c5b99228e6.

After 70b8d1c, Automake's mkdir_p is now used. Automake provides mkdir_p
via AM_INIT_AUTOMAKE since 1.7b [1]; Automake >= 1.10 provides mkdir_p too,
by wrapping around Autoconf's MKDIR_P (via AC_PROG_MKDIR_P) [2]. Thus we
need not call anything to provide mkdir_p.

[1] http://git.savannah.gnu.org/cgit/automake.git/tree/ChangeLog?id=Release-1-7b#n51
[2] http://git.savannah.gnu.org/cgit/automake.git/tree/ChangeLog?id=Release-1-9b#n219

This also benefits older Autoconfs (ie. < 2.60) which do not provide
AC_PROG_MKDIR_P.

Tested on:
 - Solaris, Autoconf 2.59, Automake 1.8.3
 - Ubuntu,  Autoconf 2.68, Automake 1.11.3