Use new configuration setting for item total class

[benjaminwil]

Summary

In our work on the in-memory order updater (#5872), we created a new configuration point item_total_class, but we did not use this configuration class in the solidus_legacy_promotions gem.

This pull request simply starts using the configuration point instead of calling the built-in Spree::ItemTotal class directly.

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[mamhoff]

Thank you, good find!

[tvdeyen]

Thanks!