Remove explicit audit event log in MembershipActions

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3536

What does this MR do?

This tries to reduce the delta between CE and EE (which you can see here: https://gitlab.com/rymai-gitlab/ce_ee_diffs_generator/blob/master/diffs/app-controllers.diff).

This removes explicit audit event log in MembershipActions and move the audit calls to specific EE modules.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together