Harmonize access level validation error in the LDAP group links API

In gitlab-org/gitlab-ce!4833 I've changed the way the access level validity is checked in the API (to rely on AR instead of a custom check) but forgot to make a similar change in the LDAP group links API.

Thankfully @vsizov fixed this by re-adding an API helper in https://gitlab.com/gitlab-org/gitlab-ee/commit/3eeb64313920e3c6df3662fcf739d0e81fd8ddda. While this does the job, I'd prefer to use the same kind of logic I've used in the members API (even though it means a bit more code and duplication for now, but the goal is to clean this ugly backward-compatible code in %9.0).