[backport#15699] Remove no-op CClientUIInterface::[signal_name]_disconnect. Disconnect BlockNotifyGenesisWait and RPCNotifyBlockChange properly.

Summary:
Disconnect BlockNotifyGenesisWait and RPCNotifyBlockChange properly. Remove no-op CClientUIInterface::[signal_name]_disconnect. (practicalswift)

Pull request description:

Resolves #15698.

Changes:
* Remove no-op `CClientUIInterface::[signal_name]_disconnect`.
* Disconnect `BlockNotifyGenesisWait` and `RPCNotifyBlockChange` properly.

Backport of Core PR15699

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7453