[backport#15458] refactor: Drop redundant wallet reference

Summary:
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

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6286