refactor: Drop CWalletRef typedef

Summary: This is extracted from Core PR13017 : https://github.com/bitcoin/bitcoin/pull/13017/commits/6efd9644cfe31168db1841010cffa64dfe604e67

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3486