CppTools: Apply 'static' pattern to StringTable

This also fixes a crash on loading (some?) projects introduced
in 577bf7c08a.

Change-Id: Ie35d466fa3b84b183118fe93f55393a4c59755de
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cpplocatordata.h

@@ -28,7 +28,6 @@
 #include "cpptools_global.h"
 #include "cppmodelmanager.h"
 #include "searchsymbols.h"
-#include "stringtable.h"
 
 #include <cplusplus/CppDocument.h>
 
@@ -64,12 +63,6 @@ private:
     void flushPendingDocument(bool force) const;
     QList<IndexItem::Ptr> allIndexItems(const QHash<QString, QList<IndexItem::Ptr>> &items) const;
 
-    QString findOrInsertFilePath(const QString &path) const
-    { return m_strings->insert(path); }
-
-private:
-    Internal::StringTable *m_strings = nullptr; // Used to avoid QString duplication
-
     mutable SearchSymbols m_search;
     mutable QHash<QString, IndexItem::Ptr> m_infosByFile;