DELETE /v3/instruments/:id/net-asset-values/:navId

Delete a specific NAV observation. Note this endpoint sits under the
instrument — not at /v3/net-asset-values/:id.

Path params:
  id    (required) — Instrument identifier.
  navId (required) — NAV identifier (the `id` field on each NAV in
                     GET /v3/instruments/:id/net-asset-values).

Response (204 No Content):
  Empty body.