C++: Remove unused var in LookupScopePrivate

Was readded by mistake when the type was pimpled.

Change-Id: I4150b783a1b54dda6070a49f78bcb8b0fcb9f414
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/LookupContext.cpp

@@ -585,7 +585,6 @@ public:
     QHash<Internal::FullyQualifiedName, Symbol *> *_scopeLookupCache;
 
     // it's an instantiation.
-    const TemplateNameId *_templateId;
     LookupScopePrivate *_instantiationOrigin;
 
     AlreadyConsideredClassContainer<Class> _alreadyConsideredClasses;