53b310390c refactor: Drop redundant wallet reference (João Barbosa)
Pull request description:
Removes the redudant wallet reference from `WalletImpl`. ```cpp // before: std::shared_ptr<CWallet> m_shared_wallet; CWallet& m_wallet; // after std::shared_ptr<CWallet> m_wallet; ```
https://github.com/bitcoin/bitcoin/pull/15458/commits/53b310390cc72a034911a8d6236b472a22c62227
Backport of Core PR15458