Refactor(eos_designs): Move management type, gateway and ipv6_gateway to metadata

[MaheshGSLAB]

in the sense that it is not rendered in the config - It should be moved to a metadata section one day with type (@MaheshGSLAB can you track this in an issue if we are not doing it already?)

        type:
          type: str
          valid_values: ["oob", "inband"]
          default: "oob"
          description: For documentation purposes only.
        gateway:
          type: str
          description: IPv4 address of default gateway in management VRF.
        ipv6_gateway:
          type: str
          description: IPv6 address of default gateway in management VRF.

these keys are only used in docuemntation tempalte today

Originally posted by @gmuloc in #6627 (comment)

[ClausHolbechArista]

let's just remove all of these in 7.0. It is unused metadata.