[qt] remove deprecated Qt:SystemLocale____Date

Summary: replace with QLocale().dateFormat(), introduced in Qt 4.1

Test Plan:

ninja all check check-functional
bitcoin-qt

check the dates look alright

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8492