[docs] QDeadlineTimer: remove docs of a function that exists no more

The _q_data() function was removed before 6.6, but the docs
remained. Presumably qdoc doesn't care about non-existing \internal
\fn's.

Found while hunting for QPair uses to eliminate.

Amends eac30fcb82.

Pick-to: 6.7 6.6
Change-Id: I73fdfe69d4158977a6a80eb2a5e790ada09fe85a
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qdeadlinetimer.cpp

@@ -748,9 +748,4 @@ QDeadlineTimer operator+(QDeadlineTimer dt, qint64 msecs)
   Returns the time remaining before the deadline.
  */
 
-/*!
-  \fn QPair<qint64, unsigned> QDeadlineTimer::_q_data() const
-  \internal
-*/
-
 QT_END_NAMESPACE