Remove inverse_of from Spree::Adjustment#order

[mamhoff]

Summary

This inverse_of gives rise to spurious adjustments appearing in the database if there are newly built and directly marked for destruction.

Since we're never calling adjustment.order anywhere in production code, I am pretty sure we can remove this inverse_of safely.

I believe what happens is that the adjustment gets added to order.all_adjustments on initialization, and marking it for destruction on the shipment does not carry on to that association as it does not have the autosave property.

Alternatively, we could add autosave to order.all_adjustments, but that could surface even weirder bugs.

Found while building #6371

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.44%. Comparing base (143f09a) to head (7ac8007).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6373   +/-   ##
=======================================
  Coverage   89.44%   89.44%           
=======================================
  Files         973      973           
  Lines       20298    20298           
=======================================
  Hits        18156    18156           
  Misses       2142     2142           

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