Drop unneeded Qt metatype declarations

KNSCore::Entry has a Q_GADGET macro and thus has automatic coverage.
KNSCore::Entry::List is a QList<Entry> typedef, where for QList<T> with T
being a registered meta type that is also registered automatically.

KNewStuffQuick::CommentsModel::IncludedComments & ItemsModel::ItemStatus
are both registered with Q_ENUM, thus also have automatic coverage.

The explicit declaration macros only add costs to all places using the
header, where the metatype declaration will be only needed by code
using the types in queued connections.