[cleanup][db] Not all datasets have associated "services" managing them

[smklein]

Context: #1954 (comment)

The dataset structure in the database describes some information which assumes each dataset will be managed by an accompanying TCP/IP-based HTTP server:

omicron/nexus/db-model/src/dataset.rs

Lines 30 to 43 in 3257446

	pub struct Dataset {
	#[diesel(embed)]
	identity: DatasetIdentity,
	time_deleted: Option<DateTime<Utc>>,
	rcgen: Generation,
	pub pool_id: Uuid,
	ip: ipnetwork::IpNetwork,
	port: SqlU16,
	kind: DatasetKind,
	pub size_used: Option<i64>,
	}

(see: ip, port).

This isn't universally true; some datasets could exist and be managed by the sled agent (for example, when setting up a local dataset for logging or other debug info).