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>
CppTools: Garbage collect if the last CppEditor is closed

Task-number: QTCREATORBUG-9828

Change-Id: Ie0ef9757cedb772702e49542b58d5b589506aa9e
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>