repo.imports: remove unused code

[pmrowla]

• [x] ❗ I have followed the Contributing to DVC checklist.

• [x] 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

Removes legacy dvc.repo.imports code (no longer used now that imports are handled the same way as everything else in dvc-data)

[codecov[bot]]

Codecov Report

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

Comparison is base (9fbe5d9) 90.42% compared to head (6731d06) 90.58%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10311 +/- ## ========================================== + Coverage 90.42% 90.58% +0.16% ========================================== Files 494 493 -1 Lines 37824 37757 -67 Branches 5470 5454 -16 ========================================== Hits 34204 34204 + Misses 2986 2919 -67 Partials 634 634 ```

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