Revert "Port to Qt 6's absence of QProcess::setupChildProcess"

This reverts commit 6a66ced594.

The patch that will introduce the alternative to setupChildProcess
(QProcess::setChildProcessModifier()) has not yet found its
way to qtbase and current Qt dev already identifies itself as
version 6.0.0, rendering the code here uncompilable.

Change-Id: I570b01598005070f0c17604226d245a3a381250e
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/qtcprocess.cpp

@@ -678,10 +678,6 @@ QtcProcess::QtcProcess(QObject *parent)
     static int qProcessProcessErrorMeta = qRegisterMetaType<QProcess::ProcessError>();
     Q_UNUSED(qProcessExitStatusMeta)
     Q_UNUSED(qProcessProcessErrorMeta)
-
-#if QT_VERSION >= QT_VERSION_CHECK(6,0,0) && defined(Q_OS_UNIX)
-    setChildProcessModifier([this]() { niceChildProcess(); });
-#endif
 }
 
 void QtcProcess::setUseCtrlCStub(bool enabled)
@@ -1224,25 +1220,18 @@ QString QtcProcess::expandMacros(const QString &str, AbstractMacroExpander *mx,
     return ret;
 }
 
-#if defined Q_OS_UNIX
-void QtcProcess::niceChildProcess()
+void QtcProcess::setupChildProcess()
 {
+#if defined Q_OS_UNIX
     // nice value range is -20 to +19 where -20 is highest, 0 default and +19 is lowest
     if (m_lowPriority) {
         errno = 0;
         if (::nice(5) == -1 && errno != 0)
             perror("Failed to set nice value");
     }
-}
-
-#  if QT_VERSION < QT_VERSION_CHECK(6,0,0)
-void QtcProcess::setupChildProcess()
-{
-    niceChildProcess();
+#endif
     QProcess::setupChildProcess();
 }
-#  endif
-#endif
 
 bool QtcProcess::ArgIterator::next()
 {