Revert "Handle the QWidgetPrivate::mapper structure"

This reverts commit 90de48493be283b9afb249f6a0fd8dbd8958517d.

The call isn't necessary, but invokes undefined behavior.

It invokes undefined behavior because deleteTLSysExtra() is called
from deleteExtra(), which is called from ~QWidgetPrivate(), which is
called from ~QObject(). Thus, by the time we call q->windowType()
within setWinId(), q is no longer a QWidget, but only a QObject, and
calling a QWidget member function then is UB.

UBSan confirms:

  qwidget_p.h:300:5: runtime error: downcast of address 0x2afdd4053620 which does not point to an object of type 'QWidget' (the Q_Q macro)
  0x2afdd4053620: note: object is of type 'QObject'
  qwidget.cpp:1712:93: runtime error: member call on address 0x2afdd4053620 which does not point to an object of type 'QWidget'
  0x2afdd4053620: note: object is of type 'QObject'

It is also unnecessary:

deleteTLSysExtra() is called from two places: QWidget::destroy() and
deleteExtra(). deleteExtra() is only called ...