Clear order promotions in Omnes event

[mamhoff]

Summary

This moves clearing the connected order promotions upon emptying an order into an Omnes::Subscriber. This saves us from having to override Spree::Order#empty! in the solidus_legacy_promotions gem.

Checklist

The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] I have used clear, explanatory commit messages.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.88%. Comparing base (098260d) to head (914d1aa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5754   +/-   ##
=======================================
  Coverage   88.87%   88.88%           
=======================================
  Files         708      709    +1     
  Lines       16825    16835   +10     
=======================================
+ Hits        14953    14963   +10     
  Misses       1872     1872           

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