CppTools: Merge diagnostic selection widgets together

QComboBox, QLabel and QPushButton are always used
together. Therefore it makes sense to put them in one
custom widget.

Change-Id: Ie21675530fbadd7071f2a362567dadb6f09bb68d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/clangdiagnosticconfigswidget.h

@@ -119,6 +119,4 @@ private:
     int m_selectedConfigIndex = 0;
 };
 
-CPPTOOLS_EXPORT void connectToClangDiagnosticConfigsDialog(QPushButton *button);
-
 } // CppTools namespace