CppTools: Move GC() call from CppEditorWidget to CppModelManager

There is no need to have that logic in CppEditorWidget. The
CppModelManager knows how many editors are open and can therefore call
GC() by itself.

Change-Id: I2ed4051ad7b8c87615014d73e29291027d49d2ab
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>