XSD: Remove display attribute from fields

[hamishwillee]

This removes the XSD to allow display="bitmask" on a field. We now expect the bitmask attribute to be set on the enum itself.

The trigger for this was the accidental reintroduction of the attribute in mavlink/mavlink#2289

My concern about this is that it might break other systems that aren't directly managed by MAVLink org.

[hamishwillee]

@peterbarker This works upstream, but we need get rid of the last instances here.

But, the question stands, what about other dialects that don't talk to this project?