gui: Remove un-actionable TODO

Summary:

With encryption turned on by default for all wallets in consideration (#18889), I believe that wallet decryption will not be implemented ever or at least any time soon. So remove that TODO comment for now. If deemed important, a brainstorming issue can be opened instead.

Also remove some TODOs in the RPC console, which I don't understand. Maybe the gui was meant to show the debug log interactively? In any case, if deemed important, this should be filed as a brainstorming feature request, so that trade-offs of different solutions can be discussed.

This is a backport of Core PR18997

Test Plan: ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9146