Unambigious ECS field: `client.domain

[janstarke]

Description of the issue:
the client structure in client.yml describes the field domain twice

Any additional context or examples:

    - name: domain
      level: core
      type: keyword
      description: >
        Client domain.

    - name: domain
      level: core
      type: keyword
      short: The domain name of the client.
      example: foo.example.com
      description: >
        The domain name of the client system.

        This value may be a host name, a fully qualified domain name, or another
        host naming format. The value may derive from the original event or be
        added from enrichment.

[ebeahan]

resolved in #2120