Remove legacy AuditEvent model and all application code referencing it

What does this MR do and why?

Removes the legacy AuditEvent model (app/models/audit_event.rb) and all application code that references it, as part of #615798 (closed).

The AuditEvent model backed the monolithic audit_events table. All new audit event writes now go to the four scoped tables via AuditEvents::GroupAuditEvent, AuditEvents::ProjectAuditEvent, AuditEvents::UserAuditEvent, and AuditEvents::InstanceAuditEvent. The legacy model is no longer written to and can be safely removed from application code.

Changes:

• Deleted app/models/audit_event.rb and ee/app/models/ee/audit_event.rb
• AuditEvents::Processor: removed the ::AuditEvent.find_by_id fallback in fetch_from_id; a nil model_class now returns nil immediately
• AuditEventPresenter: replaced presents ::AuditEvent, as: :audit_event with an explicit audit_event accessor method (the presenter is still used by all scoped models via their present methods)
• User model: removed has_many :audit_events association; removed the matching inverse_of: :audit_events from CommonModel#belongs_to :user
• Specs: deleted spec/models/audit_event_spec.rb and ee/spec/models/ee/audit_event_spec.rb
• Factory: deleted spec/factories/audit_events.rb (the legacy :audit_event factory and its sub-factory aliases :project_audit_event, :group_audit_event, :instance_audit_event, :user_audit_event); all callers updated to use the scoped :audit_events_*_audit_event factories
• EE specs: updated all specs that used create/build(:audit_event, ...) or the legacy factory aliases to use scoped factories
• Streaming shared examples: removed table-test rows that compared legacy AuditEvent records to scoped-model records
• Auditor specs: replaced AuditEvent.count assertions with AuditEventReader.count (reads across all scoped tables)

Not in scope: The audit_events table itself is NOT dropped. That is tracked separately in #512026.

References

• Closes #615798 (closed)
• Related table drop: #512026

Screenshots or screen recordings

Not applicable — no UI changes.

How to set up and validate locally

1. Run the affected specs:
   • bin/rspec spec/services/audit_events/processor_spec.rb
   • bin/rspec spec/lib/gitlab/audit/auditor_spec.rb
   • bin/rspec spec/lib/gitlab/audit/null_author_spec.rb
   • bin/rspec ee/spec/presenters/audit_event_presenter_spec.rb
   • bin/rspec ee/spec/serializers/audit_event_entity_spec.rb
   • bin/rspec ee/spec/lib/gitlab/audit/auditor_spec.rb
2. Verify the app boots: bin/rails runner "puts 'OK'"

MR acceptance checklist

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