CppTools: Remove an unused declaration

Change-Id: I165f32678d61e3731fff68982bdb3f5c1791b1f0
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/cpptools/cpplocatordata.h

@@ -36,10 +36,6 @@
 
 namespace CppTools {
 
-namespace Internal {
-class CppToolsPlugin;
-} // namespace Internal
-
 class CppLocatorData : public QObject
 {
     Q_OBJECT