Stop auto-assigning PR reviewers

[Kouteki]

Related to the comment #1007 (comment)

Current state

PRs automatically assign teams based on the CODEOWNERS file, as well as 2 core team members (chosen in round robin fashion). As a result, each core team member has 100+ PRs assigned for review.

Desired state

PR does not have anyone assigned when created. The core team and the review team assign reviewers manually. Each core team member has 2-5 PRs they are expected to review within the next 2 weeks.

To do

• Remove CODEOWNERS
• Disable round robin

[thehowl]

Whether we have round robin or not, PR reviewers are no longer assigned automatically, so closing.