Remove Dennis as extra assignee for Optimize

Summary

@wortschi is no longer interim frontend EM for Optimize, now permanent, so this removes @dennis as an extra assignee.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review