QTest: Remove pre-Qt6 code

It's disabled now, so can be deleted

Change-Id: I0d548327e7ef42bbca9ed88556bf9f8456038cc7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 4d2ef8208955846b6fca178648840b93e8fe09e1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qtestcase.cpp

@@ -1609,16 +1609,6 @@ char *toPrettyCString(const char *p, int length)
     return buffer.release();
 }
 
-#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
-// this used to be the signature up to and including Qt 5.9
-// keep it for BC reasons:
-Q_TESTLIB_EXPORT
-char *toPrettyUnicode(const ushort *p, int length)
-{
-    return toPrettyUnicode(QStringView(p, length));
-}
-#endif
-
 /*!
     \internal
     Returns the same QString but with only the ASCII characters still shown;