Use Q_DECL_OVERRIDE instead of QTC_OVERRIDE

This patch removes src/libs/utils/qtcoverride.h
and uses Q_DECL_OVERRIDE from Qt 5 instead.

Change-Id: I61ffa539648ffe996dc061eec7708c04580c0f09
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>