Remove unneeded Q_DECLARE_METATYPE calls for pointers to QObject subclasses

Compare API dox of Q_DECLARE_METATYPE:
"
Some types are registered automatically and do not need this macro:
* Pointers to classes derived from QObject
* QPointer<T>, QSharedPointer<T>, QWeakPointer<T>,
  where T is a class that derives from QObject
"

This is a ABI-compatible change, as this just replaces explicitly
defined template code (by the Q_DECLARE_METATYPE macro use) with the
generic predefined one picking up pointers to QObject subclasses.

So instead of the custom explicitly template overload
    template <>
    struct QMetaTypeId< TYPE >
    { /**/ static int qt_metatype_id() {/**/} };
the predefined template overload one is picked up via
    template <typename T>
    struct QMetaTypeId : public QMetaTypeIdQObject<T>
    { /**/ };
    template <typename T, int =
        QtPrivate::IsPointerToTypeDerivedFromQObject<T>::Value ?
            QMetaType::PointerToQObject :
        /**/ 0>
    struct QMetaTypeIdQObject
    { /**/ };
    template <typename T>
    struct QMetaTypeIdQObject<T*, QMetaType::PointerToQObject>
    { /**/ static int qt_metatype_id() {/**/} };

And thus the constexpr code using QMetaTypeId<T>::qt_metatype_id(),
for which all this is done, evaluates as before.

GIT_SILENT

src/plasma/containmentactions.h

@@ -172,6 +172,4 @@ private:
     K_PLUGIN_FACTORY_WITH_JSON(factory, jsonFile, registerPlugin<classname>();) \
     K_EXPORT_PLUGIN_VERSION(PLASMA_VERSION)
 
-Q_DECLARE_METATYPE(Plasma::ContainmentActions *)
-
 #endif // PLASMA_CONTAINMENTACTIONS_H

src/plasma/dataengine.h

@@ -487,6 +487,4 @@ private:
     K_PLUGIN_FACTORY_WITH_JSON(factory, jsonFile, registerPlugin<classname>();) \
     K_EXPORT_PLUGIN_VERSION(PLASMA_VERSION)
 
-Q_DECLARE_METATYPE(Plasma::DataEngine *)
-
 #endif // multiple inclusion guard

src/plasma/private/storage_p.h

@@ -54,9 +54,6 @@ private:
 };
 //End StorageJob
 
-Q_DECLARE_METATYPE(StorageJob *)
-Q_DECLARE_METATYPE(QPointer<StorageJob>)
-
 class Storage : public Plasma::Service
 {
     Q_OBJECT

src/plasma/service.h

@@ -223,8 +223,6 @@ private:
 
 } // namespace Plasma
 
-Q_DECLARE_METATYPE(Plasma::Service *)
-
 /**
  * Register a service when it is contained in a loadable module
  */

src/plasma/servicejob.h

@@ -122,7 +122,5 @@ private:
 
 } // namespace Plasma
 
-Q_DECLARE_METATYPE(Plasma::ServiceJob *)
-
 #endif // multiple inclusion guard
 

src/scriptengines/qml/plasmoid/appletinterface.cpp

@@ -44,8 +44,6 @@
 #include <kdeclarative/configpropertymap.h>
 #include <kdeclarative/qmlobject.h>
 
-Q_DECLARE_METATYPE(AppletInterface *)
-
 AppletInterface::AppletInterface(DeclarativeAppletScript *script, const QVariantList &args, QQuickItem *parent)
     : AppletQuickItem(script->applet(), parent),
       m_configuration(nullptr),