Remove `rusqlite::Connection`-based impl of `WalletRead`

[str4d]

This seems pretty unsafe to me, as get_wallet_summary is likely to be called from multiple threads; I think that we need to remove this impl entirely; that should be a separate PR (or an additional commit to this PR) before release.

Originally posted by @nuttycom in https://github.com/zcash/librustzcash/pull/1122#discussion_r1466576780