remaining "purpose" word

[Kosuke Koiwai]

https://openid.bitbucket.io/ekyc/openid-connect-4-identity-assurance.html#section-5.3-4

The object may contain restrictions using value or values as defined in [OpenID] and/or the essential or purpose keys as described below. An example is shown in the following:

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the purpose above should have been deleted with https://bitbucket.org/openid/ekyc-ida/pull-requests/185