Remove duo_workflow_workhorse feature flag

What does this MR do and why?

Remove duo_workflow_workhorse feature flag. It has been enabled by default and enabled globally on .com

Related issue:

• [FF] `duo_workflow_workhorse` -- Control if Wor... (#547503 - closed)
• [FF] `duo_workflow_web_chat_mutation_tools` -- ... (#550853 - closed)