CMakePM: Remove <Headers> virtual project node

CMake projects need to add the headers as source files in order to get
exported from the CMake file-api json export.

Having header files that are not part of the project displayed in the
<Headers> node is error prone and confusing.

This also means that you won't get bogus files when doing git commit
for example.

Fixes: QTCREATORBUG-18206
Fixes: QTCREATORBUG-24609
Fixes: QTCREATORBUG-25407
Change-Id: I89ac4f8a80f452119f8a991b9e4ef14efb7a86b9
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/fileapidataextractor.h

@@ -67,7 +67,6 @@ public:
     QList<CMakeBuildTarget> buildTargets;
     ProjectExplorer::RawProjectParts projectParts;
     std::unique_ptr<CMakeProjectNode> rootProjectNode;
-    QSet<Utils::FilePath> knownHeaders;
     QString ctestPath;
     bool isMultiConfig = false;
     bool usesAllCapsTargets = false;