chore(ci): remove auto-request-reviews action

this commit is a followup to 25d691d (#4448), where in we
removed/commented out the events tied to this action. the hope in that
commit was to temporarily disable auto request reviews while we worked
through round robin testing.

however, this action still attempts to run on merge queue actions (more
specifically, GH parses the action and see that it doesn't have an event
`on` section and sends an error email). to put a stop to that, remove
the action.

should we decide to re-enable this action, this and 25d691d should be
reverted in the same commit/pr