Remove dap_schema_auto_tool_choice feature flag

What does this MR do and why?

Removes the dap_schema_auto_tool_choice feature flag from the monolith. The AI gateway stopped reading the flag in ai-assist!6950, where tool_choice="auto" for response schemas became the default, so there is nothing left for the monolith to gate.

Closes the follow-up tracked in #628102.

How to set up and validate locally

1. Check out this branch.
2. Confirm no references remain: grep -rn "dap_schema_auto_tool_choice" .
3. Run bundle exec rspec ee/spec/lib/api/helpers/duo_workflow_helpers_spec.rb

MR acceptance checklist

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