walletdb: remove fAggressive from Salvage

Summary:

The only call to Salvage set fAggressive = true so remove that parameter
and always use DB_AGGRESSIVE

Partial backport (5/11) of core PR18918:
https://github.com/bitcoin/bitcoin/pull/18918/commits/07250b8dcebe2b97ed0fd900ad35cba4091b8ecf

Depends on D8591.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8592