[CE] Exclude LDAP from OmniauthCallbackController base methods

What

Backports changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5449

Removes methods from OmniauthCallbackController which will be defined in subcontrollers instead

Why was this MR needed?

Refactoring

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?