[Federation] Remove duplicate normative text about custom Entity Statement claims

Issue #2072 resolved
Vladimir Dzhuvinov created an issue

In https://openid.bitbucket.io/connect/openid-federation-1_0.html#name-entity-statement

we have

Other claims MAY be defined and included.

Applications and protocols utilizing Entity Statements MAY specify additional claims.

The 1st sentence was brought back from an old spec version after some merge conflict resolution. It should be removed.