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