btrfs/118: remove call to _need_to_be_root

btrfs/118: remove call to _need_to_be_root

The function _need_to_be_root does not exists anymore as of commit
56ff01f471c9 ("xfstests: remove _need_to_be_root").

A v2 of the patch that added test btrfs/118 without calling this
function was sent but not picked [1], instead v1 was picked.

So fix this now.

[1] https://patchwork.kernel.org/patch/8354831/

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