QbsProject pushes duplicated include paths to the code model

1. Open qtcreator.qbs
2. Open e.g. clangcompletionassistprocessor.h
3. At the top, start writing #include <cpptool
   ==> The resulting completion list yields multiple "cpptools/" entries.

This is due duplicated include paths. In QbsProject::updateCppCodeModel the call ppBuilder.setHeaderPaths(grpHeaderPaths); sets the duplicated include paths.

Side note: To check the currently used include path for the code model for the currently open file: Menu: Tools > C++ > "Inspect C++ Code Model", select top level tab "Project Parts", select bottom level tab "Header Paths".