Remove redundant WHERE from event queries

What does this MR do?

This removes a redundant WHERE events.author_id IS NOT NULL filter from Event's default scope. This is redundant because this column can not have any NULL values.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/35586