kv: delete `Internal` service and its references post-gRPC elimination

The DRPC migration has provided an excellent opportunity to refactor the `Internal` service into more focused and logical smaller services. Once the DRPC migration is complete and gRPC is fully removed, we must ensure that the `Internal` service itself is also deleted from the proto definitions.

For context on the strategy for splitting the `Internal` service, refer to https://github.com/cockroachdb/cockroach/pull/147063. An example of one such extraction can be found at https://github.com/cockroachdb/cockroach/pull/145195.

Jira issue: CRDB-51621