[qt] remove unused parameter from getWarnings()

Summary:
This is a partial backport of Core PR17750 [1/3]
https://github.com/bitcoin/bitcoin/pull/17750/commits/869b6314fd180856b6054fff28b5de994252c54c

Test Plan:
ninja && src/qt/bitcoin-qt -regtest

The warning banner "This is a pre-release test build..." uses getWarnings().

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8329