Remove instance variable assignment for Services.oidc
Remove instance variable assignment for Services.oidc

This has no effect.  Module "instance variables" are assigned in any
class which includes the module, but this module isn't included
anywhere.

This just caused me some confusion, as I was trying to work out if the
`OidcClient` instance would persist between requests.  It won't.  So
remove the confusing `@`.

alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend