btrfs: stop setting log_root_tree->log_root to NULL in btrfs_recover_log_trees()

There's no point in setting log_root_tree->log_root to NULL as this is
already NULL, we never assigned anything to it before and it's meaningless
as a log root never has a value other than NULL for the ->log_root field,
that can be not NULL only for non log roots.

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