QProcess/Unix: remove outdated notices about use of posix_spawn() on QNX

We haven't used the spawn functionality on QNX since Qt 5.7 (commit
005a8bfbf0022f03dafafcf2b5c438ccf0675a49) because that's when we dropped
support for QNX 6.5.0.

Change-Id: Ic90d8429a0eb4837971dfffd1664f9712bdce2d8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

src/corelib/io/qprocess_unix.cpp

@@ -504,10 +504,6 @@ void QProcessPrivate::startProcess()
         delete [] envp;
     }
 
-    // On QNX, if spawnChild failed, childPid will be -1 but forkfd is still 0.
-    // This is intentional because we only want to handle failure to fork()
-    // here, which is a rare occurrence. Handling of the failure to start is
-    // done elsewhere.
     if (forkfd == -1) {
         // Cleanup, report error and return
 #if defined (QPROCESS_DEBUG)

tests/auto/corelib/io/qprocess/tst_qprocess.cpp

@@ -2260,9 +2260,6 @@ void tst_QProcess::setNonExistentWorkingDirectory()
     QCOMPARE(int(process.error()), int(QProcess::FailedToStart));
 
 #ifdef Q_OS_UNIX
-#  ifdef QPROCESS_USE_SPAWN
-    QEXPECT_FAIL("", "QProcess cannot detect failure to start when using posix_spawn()", Continue);
-#  endif
     QVERIFY2(process.errorString().startsWith("chdir:"), process.errorString().toLocal8Bit());
 #endif
 }