Remove expires Value as Causing Issues with OAuth2

See https://github.com/intridea/oauth2/blob/cd8a62e69cab6778fe9eae6637719e5cbfe1cff3/lib/oauth2/access_token.rb

`@expires_in` gets set to the value of the `expires` key, which from omniauth-oauth2 is a boolean. It then tries to convert this to an integer and fails.

If `expires_in` was also passed from omniauth-oauth2 then this would be fine, but only `expires_at` is passed.