[nit] do not capture unused this in wallet interface

Summary:
Remove capture of this in lambda within wallet.cpp to fix the following warning:

../../src/interfaces/wallet.cpp:405:22: warning: lambda capture 'this' is not used [-Wunused-lambda-capture]
                [fn, this](CWallet *, const TxId &txid, ChangeType status) {

Test Plan:

make

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3166