Flat Rate calculator: Do not override #compute

[mamhoff]

Summary

Spree::Calculator#compute dispatches to compute_line_item, compute_shipment or compute_shipping_rate, and we want to keep that behavior. This also documents what kind of objects this calculator knows how to reasonably work with.

Extracted from #6287

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (2d1fcff) to head (b7f5ed4).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6349   +/-   ##
=======================================
  Coverage   89.34%   89.34%           
=======================================
  Files         961      961           
  Lines       20165    20168    +3     
=======================================
+ Hits        18016    18019    +3     
  Misses       2149     2149           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.