Deleting a link shouldn't fail if the transceiver isn't present

[bnaecker]

Dendrite's link reconciler attempts to match the configured and "plumbed" settings for each link. One of the first things it does is try to fetch transceiver's MPN here:

dendrite/dpd/src/link.rs

Lines 1785 to 1799 in 2475028

	if let Some(qsfp) = &qsfp {
	match qsfp_xcvr_mpn(qsfp) {
	Ok(mpn) => Some(mpn),
	Err(e) => {
	warn!(log, "failed to get MPN for qsfp";
	"port" => %port_id,
	"error" => %e,
	);
	return;
	}
	}
	} else {
	None
	}
	};

That fails with Err(DpdError::Missing) if the transceiver isn't present. That's reasonable when trying to create a link, but it blocks deleting a link if the transceiver has been unplugged. We should continue attempting to delete the link in this case.