kv: Remove unused impl of EngineIterator for DBIterator (#9507)
kv: Remove unused impl of EngineIterator for DBIterator (#9507)

Signed-off-by: Brian Anderson <2709ac8f6d3dc60d22e49a091caa2d2f10738e41@gmail.com>

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### What problem does this PR solve?

Cleaning up the tikv::storage::kv module to make it eventually generic over engine_traits

- cc https://github.com/tikv/tikv/issues/9480
- cc https://github.com/tikv/tikv/issues/6402

### What is changed and how it works?

This code is unused. Delete it.

### Related changes


### Check List <!--REMOVE the items that are not applicable-->


### Release note <!-- bugfixes or new feature need a release note -->

- No release note.
c4pt0r/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv