Stop adding ?schema=openid to userinfo endpoint URL.

[gkreitz]

Stops appending a fixed (afaict, non-standard) query string to the userinfo endpoint.

Fixes #448
Fixes #388

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[ricklambrechts]

The schema=openid is indeed not part of the Userinfo request anymore.
https://openid.net/specs/openid-connect-core-1_0.html#UserInfoRequest

I think it was part of an earlier specification. In the notices of the basic specification https://openid.net/specs/openid-connect-basic-1_0.html, there is a notice:

Fixed #801 - Removed schema and id parameters to UserInfo Endpoint. Also fixed related issue #791 - Removed invalid_schema error.