Move shipping promotion handling to legacy promotions

[mamhoff]

Summary

We should only provide configuration points that are actually necessary for a promotion system. The shipping promotion handler is not necessary for a promotion system that works, thus we can move this stuff to the legacy_promotions extension.

Extracted from #5634

Checklist

Check out our PR guidelines for more details.

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.

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.