qt: Avoid resetting on resetguisettigs=0

Summary: Backport from core PR12793

Test Plan:

./src/qt/bitcoin-qt -resetguisettings=0

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2195