Avoid copying CWalletTx in LoadToWallet

Summary:
The change in walletdb.cpp is easier to review ignoring whitespace.

This change is need to get rid of CWalletTx copy constructor.

This is a backport of Core PR9381 [3/5]
https://github.com/bitcoin/bitcoin/pull/9381/commits/65b9d8f8ddb5a838454efc8bdd6576f0deb65f6d
Depends on D9075

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9076