btrfs: update comment for fs_devices::seed_list in btrfs_rm_device

Update it since commit 944d3f9f ("btrfs: switch seed device to
list api") did conversion from fs_devices::seed to fs_devices::seed_list.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Su Yue <l@damenly.su>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>