Get rid of unneeded CWalletTx::Init parameter

Summary:
This is a backport of Core PR9381 [2/5]
https://github.com/bitcoin/bitcoin/pull/9381/commits/bd2fbc7cdbec46400341209f4cb7e69e5b2cee19
Depends on D9074

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9075