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

cc @athal7 who suggested this fix.
nice.  i don't think we need the `where(adjustable_type: "Spree::LineItem")` condition.  i think it makes sense that this method is just clearing out all tax adjustments.