CppTools: Use fallback project part if project is closed

...instead of sticking to the old one.

If a project is closed, the editor should reflect that. It also simplifies the code.

Change-Id: I9b8a94513d3b06a238cfc4ee60c11c0d0da3d93f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppprojectpartchooser.h

@@ -50,9 +50,8 @@ public:
 
     ProjectPartInfo choose(
             const QString &filePath,
-            const ProjectPart::Ptr &currentProjectPart,
+            const ProjectPartInfo &currentProjectPartInfo,
             const ProjectPart::Ptr &manuallySetProjectPart,
-            bool stickToPreviousProjectPart,
             const ProjectExplorer::Project *activeProject,
             Language languagePreference,
             bool projectHasChanged) const;