Clang: Disable PCHs generation

Some important features are missing so it should be disabled.

Change-Id: I24f8f2cd42c6c4d7152a36db0fe8496e4fb824a6
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangpchmanager/clangpchmanagerplugin.cpp

@@ -65,9 +65,6 @@ public:
     PrecompiledHeaderStorage<> preCompiledHeaderStorage{database};
     PchManagerClient pchManagerClient{preCompiledHeaderStorage};
     PchManagerConnectionClient connectionClient{&pchManagerClient};
-    QtCreatorProjectUpdater<PchManagerProjectUpdater> projectUpdate{connectionClient.serverProxy(),
-                                                                    pchManagerClient,
-                                                                    filePathCache};
 };
 
 std::unique_ptr<ClangPchManagerPluginData> ClangPchManagerPlugin::d;