qt gui init: rm deprecated QApplication attr AA_X11InitThreads

Summary:
From https://doc.qt.io/qt-5/qt.html :

The following values are deprecated or obsolete:
Constant Value Description
Qt::AA_X11InitThreads 10 This value is obsolete and has no effect.

We started setting this attribute back when using Qt4,
as a workaround for some threading issues: https://github.com/spesmilo/electrum/pull/2820

However,

• in Qt5 it no longer has any effect,
• the original threading issues were fixed in https://github.com/spesmilo/electrum/pull/3326 and https://github.com/spesmilo/electrum/pull/3330

This is a backport of https://github.com/spesmilo/electrum/commit/f2e3d1101481e57711ba5fbbb37f733aa6b51298

Test Plan: run the application

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18309