Docs: remove remark of registering custom types

Types are automatically registered these days, so the remark about
explicitly registering them is out of date and can be removed.

Pick-to: 6.8
Change-Id: I9c2b4dde8728a1d884ea37f00de919a037b6dca5
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/testlib/qtestcase.cpp

@@ -2627,9 +2627,6 @@ QTestData &QTest::addRow(const char *format, ...)
     Example:
     \snippet code/src_qtestlib_qtestcase.cpp 21
 
-    To add custom types to the testdata, the type must be registered with
-    QMetaType via \l Q_DECLARE_METATYPE().
-
     \note This function can only be used called as part of a test's data
     function that is invoked by the test framework.