[VxWorks] qthread_unix contains DKM related code

DKM has not been supported in Qt for VxWorks 5.15 and not it 6.x, all the DKM related code should be removed.

#if defined(Q_OS_VXWORKS) && defined(VXWORKS_DKM)
    // for other scheduling policies than SCHED_RR or SCHED_FIFO
    prio_min = SCHED_FIFO_LOW_PRI;
    prio_max = SCHED_FIFO_HIGH_PRI;    if ((*sched_policy == SCHED_RR) || (*sched_policy == SCHED_FIFO))
#endif
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