Remove Security Manager visibility gate from GraphQL enums

What does this MR do and why?

SecurityManagerConfig.enabled? returns a literal true, so the visible? blocks on the SECURITY_MANAGER value in AccessLevelEnum and GitlabSubscriptionsUserRole always return true.

Both values are already present in the dumped schema, so there is no schema churn here. Verified with the same two tasks the graphql CI job runs.

bundle exec rake gitlab:graphql:check_docs
bundle exec rake gitlab:graphql:check_introspection_sync

Both report up to date, so doc/api/graphql/reference/_index.md and the introspection results need no regeneration.

How to set up and validate locally

bundle exec rspec spec/graphql/types/access_level_enum_spec.rb ee/spec/graphql/types/gitlab_subscriptions/user_role_enum_spec.rb ee/spec/graphql/resolvers/members/standard_roles_resolver_spec.rb

7 examples, 0 failures.

References

#584144 (closed)

Part of the work in !239507 (closed)

Screenshots or screen recordings

No UI change.

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