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