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/ssh/sshprocess.cpp

@@ -47,12 +47,6 @@ SshProcess::SshProcess()
             env.set("DISPLAY", ":0");
     }
     setProcessEnvironment(env.toProcessEnvironment());
-
-#if QT_VERSION >= QT_VERSION_CHECK(6,0,0) && defined(Q_OS_UNIX)
-    setChildProcessModifier([]() {
-        setsid(); // Otherwise, ssh will ignore SSH_ASKPASS and read from /dev/tty directly.
-    });
-#endif
 }
 
 SshProcess::~SshProcess()
@@ -68,13 +62,11 @@ SshProcess::~SshProcess()
     waitForFinished(1000);
 }
 
-#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
 void SshProcess::setupChildProcess()
 {
 #ifdef Q_OS_UNIX
     setsid(); // Otherwise, ssh will ignore SSH_ASKPASS and read from /dev/tty directly.
 #endif
 }
-#endif
 
 } // namespace QSsh