Exclude GroupSAML from sign in buttons

What

• Removes Sign In button for Group SAML
• Removes methods from OmniauthCallbackController which will be defined in subcontrollers instead

Why

Group level SAML can't be used to sign in, and if it could would need to happen for a specific group instead of on the main sign in page.

Why not

• This is WIP until the subcontroller refactor merges as I've also removed LDAP from that controller
• I'll create a CE MR for the LDAP auth controller changes

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Backend
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?