Bug 891424. Directly remove nsCocoaWindow's from their parent in Destroy(). This allows us to stop implementing GetParent because the mac widget code has some assumptions about that always returning null. r=smichaud

This reverts implementing GetParent on nsCocoaWindow's (ie back out bug 869151) because the mac widget code has some assumptions about that. Instead we just remove nsCocoaWindow's from their mParent in Destroy() which will still keep bug 869151 fixed.