remove unused fnoncriticalerrors variable from cwalletdb::findwallettx

Summary:
remove unused fnoncriticalerrors variable from cwalletdb::findwallettx

Backport of Core PR11923
https://github.com/bitcoin/bitcoin/pull/11923/files

Completes T570

Test Plan:
make check
test_runner.py
bitcoin-qt

Reviewers: Fabien, jasonbcox, deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2742