Removed provider_id from Vehicle - Register

This was discussed on working group calls and decided to be defined as part of a larger change in a forthcoming release due to security/abuse concerns noted in [this conversation](openmobilityfoundation/mobility-data-specification#469 (comment)).

agency/README.md

@@ -108,7 +108,6 @@ Body Params:
 | `vehicle_id` | String | Required | Vehicle Identification Number (vehicle_id) visible on vehicle |
 | `vehicle_type` | Enum | Required | [Vehicle Type][vehicle-types] |
 | `propulsion_types` | Enum[] | Required | Array of [Propulsion Type][propulsion-types]; allows multiple values |
-| `provider_id`| UUID | Optional | Provider to which the vehicle belongs if different from the authenticated provider. See MDS [provider list](/providers.csv). |
 | `year` | Integer | Optional | Year Manufactured |
 | `mfgr` | String | Optional | Vehicle Manufacturer |
 | `model` | String | Optional | Vehicle Model |