Draft: Drop duo_agent_platform_request_count from ai_settings

What does this MR do and why?

Drop duo_agent_platform_request_count from ai_settings

The duo_agent_platform_request_count column was ignored in 18.6 and can now be dropped in 18.7 according to the deprecation policy.

The column was originally added to ai_settings but the functionality was moved to namespace_settings instead.

Changelog: changed Closes: #579366

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Related to #579366