DiskBlockIndex: remove unused ToString() class member

Summary:
and mark its inherited CBlockIndex#ToString public interface member
as deleted, to disallow calling it in the derived CDiskBlockIndex class.

This is a partial backport of core#25349
https://github.com/bitcoin/bitcoin/pull/25349/commits/99e8ec8721a52cd08bdca31f6e926c9c1ce281fb

Depends on D16070

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16071