btrfs: remove free space cache inode check in btrfs_truncate_inode_items

We no longer have inode cache feature, so this check is extraneous as
the only inode cache is in the tree_root, which is not marked as
SHAREABLE.

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