remove krufted shielded_balance parameter

[zancas]

This PR started life as part of my review for #1192 but this seemed like a landable chunk of improvement along the way.

The target_addr parameter was being passed None in all cases.

I suspect it pre-dated the filter abstraction.

Also in this PR:

• doc-comment for wallet/describe.rs
• super --> crate::wallet in wallet/describe.rs

Creates a trivial conflict with #1192

[zancas]

Superseded by #1195