[fix] replace leftover dustRelayFee symbols in wallet/wallet.cpp
[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 [[https://github.com/bitcoin/bitcoin/pull/15638 | 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

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