akonadi: fix some clazy warnings

Summary:
relation.h:30:1: warning: Move qHash(Akonadi::Relation) into Akonadi namespace for ADL lookup [-Wclazy-qhash-namespace]
itemcreatehandlertest.cpp:874:30: warning: Don't call QVector::operator[]() on temporary [-Wclazy-detaching-temporary]
tagmodeltest.cpp:266:5: warning: unused QList<QVariant> (removedTagList)

Test Plan: builds, ctest passes

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D20714