netlink: specs: ovs: remove fixed header fields from attrs

Op's "attributes" list is a workaround for families with a single
attr set. We don't want to render a single huge request structure,
the same for each op since we know that most ops accept only a small
set of attributes. "Attributes" list lets us narrow down the attributes
to what op acctually pays attention to.

It doesn't make sense to put names of fixed headers in there.
They are not "attributes" and we can't really narrow down the struct
members.

Remove the fixed header fields from attrs for ovs families
in preparation for C codegen support.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>