gui: Drop unused return values in WalletFrame

Summary:
This is a backport of Core PR15464

addWallet is not modified because the return value is added back and used in PR16963 which was backported in D7699.

Test Plan: ninja && src/qt/bitcoin-qt

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8363