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 Qt::AA_X11InitThreads 10      This value is obsolete and has no effect.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

electrum/gui/qt/__init__.py

@@ -119,7 +119,6 @@ class ElectrumGui(BaseElectrumGui, Logger):
         # GC-ed when windows are closed
         #network.add_jobs([DebugMem([Abstract_Wallet, SPV, Synchronizer,
         #                            ElectrumWindow], interval=5)])
-        QtCore.QCoreApplication.setAttribute(QtCore.Qt.AA_X11InitThreads)
         if hasattr(QtCore.Qt, "AA_ShareOpenGLContexts"):
             QtCore.QCoreApplication.setAttribute(QtCore.Qt.AA_ShareOpenGLContexts)
         if hasattr(QGuiApplication, 'setDesktopFileName'):