automake: complete the removal of the target package

The variable ACLOCAL_DIR points to the staging area but is not useful
anymore because the target package was removed.  Fix ACLOCAL_DIR by
pointing to $(HOST_DIR)/usr/share/aclocal and remove ACLOCAL_HOST_DIR
because ACLOCAL_DIR has the same value so it's not useful anymore.

Also remove the reference to ACLOCAL_HOST_DIR from the host-autoconf
package.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>