Deleting external subnets is overly-complicated

[bnaecker]

The code for deleting external subnets looks like this:

omicron/nexus/db-queries/src/db/datastore/external_subnet.rs

Lines 544 to 565 in 16c22eb

	pub async fn delete_external_subnet(
	&self,
	opctx: &OpContext,
	authz_subnet: &authz::ExternalSubnet,
	) -> DeleteResult {
	opctx.authorize(authz::Action::Delete, authz_subnet).await?;
	let id = authz_subnet.id().into_untyped_uuid();
	delete_external_subnet_query(&id)
	.execute_async(&*self.pool_connection_authorized(opctx).await?)
	.await
	.map_err(decode_delete_external_subnet_error)
	.and_then(|count| {
	if count == 0 {
	Err(Error::not_found_by_id(
	ResourceType::ExternalSubnet,
	&id,
	))
	} else {
	Ok(())
	}
	})
	}

That delegates to a custom query, which checks that the subnet isn't attached to an instance before deleting it. This is much more complicated than in needs to be. I forgot about the UpdateAndCheck trait we have for exactly this kind of conditional delete. The code is correct, but we should just use the standard mechanism.