ProjectExplorer: Make Project::id value based

Change-Id: Ie210d2a068158c6caaac66aae58cbd886f61c18d
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/qmlprojectmanager/qmlproject.h

@@ -61,7 +61,6 @@ public:
     QString filesFileName() const;
 
     QString displayName() const;
-    Core::Id id() const;
     Core::IDocument *document() const;
     ProjectExplorer::IProjectManager *projectManager() const;