btrfs: remove log_extents_lock and logged_list from struct btrfs_root

The logged_list[2] and log_extents_lock[2] members of struct btrfs_root
are no longer used, their last use was removed in commit 5636cf7d


("btrfs: remove the logged extents infrastructure"). So remove these
fields. This reduces the size of struct btrfs_root, on a release kernel,
from 1392 bytes down to 1352 bytes.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>