Make use of or remove unused fields in InstanceProperties and Instance types

[gjcolombo]

These structs are part of the API and have long-unused fields. In InstanceProperties the image_id and bootrom_id fields are unused:

propolis/crates/propolis-api-types/src/lib.rs

Lines 198 to 217 in 99fa564

	#[derive(Clone, Debug, Deserialize, PartialEq, Eq, Serialize, JsonSchema)]
	pub struct InstanceProperties {
	/// Unique identifier for this Instance.
	pub id: Uuid,
	/// Human-readable name of the Instance.
	pub name: String,
	/// Free-form text description of an Instance.
	pub description: String,
	/// Metadata used to track statistics for this Instance.
	pub metadata: InstanceMetadata,
	/// ID of the image used to initialize this Instance.
	pub image_id: Uuid,
	/// ID of the bootrom used to initialize this Instance.
	pub bootrom_id: Uuid,
	/// Size of memory allocated to the Instance, in MiB.
	pub memory: u64,
	/// Number of vCPUs to be allocated to the Instance.
	pub vcpus: u8,
	}

And in Instance (the type returned by GET operations) the disks and nics fields are unused:

propolis/crates/propolis-api-types/src/lib.rs

Lines 226 to 233 in 99fa564

	#[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
	pub struct Instance {
	pub properties: InstanceProperties,
	pub state: InstanceState,
	pub disks: Vec<DiskAttachment>,
	pub nics: Vec<NetworkInterface>,
	}

These fields should either be used or removed. (In the latter case I'm uncertain whether or how the control plane would use this information.)