[fix] replace leftover dustRelayFee symbols in wallet/wallet.cpp

Summary:
I messed up D5943 too. I failed to replace three references to the
dustRelayFee global by the chain().relayDustFee() getter, which are
blocking proper backporting of PR15638 now that D6710 is landed

Test Plan:

ninja check-all [-DBUILD_BITCOIN_WALLET=OFF as well]

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6792