qt gui __init__: rm deprecated QApplication attr AA_X11InitThreads
qt gui __init__: rm deprecated QApplication attr AA_X11InitThreads

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 Qt::AA_X11InitThreads 10      This value is obsolete and has no effect.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

pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,pooler/electrum-ltc