qfloat16: simplify the legacy EXTERN_TAGGED content and mark for removal

Amends 21950e3085bd45dbd4b55fb76e3d47319cbb11d7.

This functionality was an ill-advised attempt at reducing compilation
times that added more legacy we have to keep until 7.0, instead of
properly fixing QMetaTypes.

qfloat16 is a built-in type whose name in the macro and the one obtained
from C++ match ("qfloat16"), and has no legacy operations to be
registered. This function only needs to return the ID.

Change-Id: I2a8300dd44bcfc1a2479fffd2f76964206b9e58a
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/global/qfloat16.cpp

@@ -10,10 +10,14 @@
 #include <QtCore/qmetatype.h>
 #include <QtCore/qtextstream.h>
 
-QT_DECL_METATYPE_EXTERN(qfloat16, Q_CORE_EXPORT)
 QT_BEGIN_NAMESPACE
 
-QT_IMPL_METATYPE_EXTERN(qfloat16)
+#if QT_VERSION < QT_VERSION_CHECK(7, 0, 0)
+Q_DECL_EXPORT int qRegisterNormalizedMetaType_qfloat16(const QByteArray &)
+{
+    return QMetaType::Float16;
+}
+#endif
 
 /*!
     \class qfloat16