Merge #15458: refactor: Drop redundant wallet reference

53b3103 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;
  ```

Tree-SHA512: c78c5a75a59df20b1684f746305c5b3505f73adfd84f00fd800da61ca07c6a37a992fedea3da390afd9564dc6b6a022d183a38995124783ad62c0cc7610be92b

Author: MarcoFalke