Remove obsolete_counts from grub2 compatibility list

PR#15459 add all read-only compatible zpool features to grub2
compatibility list. 'obsolete_counts' is a read-only features that
depends on 'device_removal' feature which is not read-only and
is marked as ZFEATURE_FLAG_MOS. Creating a pool with grub2
compatibility enables 'device_removal' feature as well, which is
not desired.

This commit removes the 'obsolete_counts' feature from
grub2 compatibility list, as GRUB only supports read-only
compatible features.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #15499