Delete stale tax adjustments, even when their line items are missing

This shouldn't happen, but we currently do have some adjustments whose
line items have disappeared. This will include getting rid of those,
also.

This will fix
https://trello.com/c/soEC8z4P/49-spree-backend-error-nomethoderror-undefined-method-persisted-for-nil-nilclass
and
https://www.honeybadger.io/inspector/p/33720/5895315/22#comment_1471956
Delete all tax adjustments before recalculating