[backport#16349] Remove redundant WalletController::addWallet slot

Summary:
https://github.com/bitcoin/bitcoin/pull/16349/commits/6285a318d77dbfdf50f893963ebfb2973746f757

Depends on D6183

Backport of Core PR16349

Test Plan:

ninja check
./srt/qt/bitcoin-qt -regtest

check the UI works as expected, close the default wallet, open the default wallet

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6184