Remove QMetaType::unregisterType().

Remove QMetaType::unregisterType().

The function hasn't been working properly. It was not well tested, for
example it is undefined how QVariant should behave if it contains an
instance of an unregistered type.

Concept of unregistering types was inspired by plug-in system, but in
most supported platforms we do not unload plug-ins.

Idea of type unregistering may block optimizations in meta object
system, because it would be not possible to cache a type id.
QMetaType::type() could return different ids for the same name.

Currently QMetaType::unregisterType() is not used in Qt.

Change-Id: I878b6e8d91de99f9bcefeab73af2e2ba0bd0cba0
Reviewed-by: Prasanth Ullattil <prasanth.ullattil@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>