[backport#15588 1/3] Remove nFileVersion from CWalletScanState

Summary:
nFileVersion is not the actual file version and is not used except
in one place. So it is removed from CWalletScanState and changed so
that it is just read at the place it is needed. Furthermore, the
"version" record now only indicates the version of the highest
versioned client that has opened a wallet file so the variable
name is changed accordingly

https://github.com/bitcoin/bitcoin/pull/15588/commits/c88e87c3b2be3f97b712107e04285d06dfef3878

Partial backport of Core PR15588

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7017