DUChain: Remove duplicates inside getInheriters

Summary:
Example navigation popup before:

Inheritors for AbstractDeclarationNavigationContext:
DeclarationNavigationContext, CMakeDeclarationNavigationContext, QmlJS::DeclarationNavigationContext,
DeclarationNavigationContext

After:

Inheritors for AbstractDeclarationNavigationContext:
CMakeDeclarationNavigationContext, QmlJS::DeclarationNavigationContext,
DeclarationNavigationContext

Reviewers: mwolff

Reviewed By: mwolff

Subscribers: mwolff, kdevelop-devel

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D664