Remove. We weren't using the fields that made this different from

Remove. We weren't using the fields that made this different from

	* camel-mime-body-part.[ch]: Remove. We weren't using the fields
	that made this different from camel-mime-part, so it basically
	just forced us to do lots of gratuitous typecasting.

	* camel-multipart.[ch]: Use CamelMimePart. Remove the multipart
	parent stuff, since we weren't using that either.

	* etc: update for CamelMimeBodyPart -> CamelMimePart