minor simplification of v3.1 schema (#2671)

* minor simplification of v3.1 schema

"if the X property is present, then apply this constraint to property X" can
be simplified by simply stating the constraint for property X.

* add missing constraint to "header"

https://spec.openapis.org/oas/v3.1.0#headerObject

* allowEmptyValue and allowReserved don't make sense for headers

* collapse single-item "enum"s in "cookie" and "header" to "const"