Draft: Cleanup showDuoAgentPlatformEnabledSetting logic

What does this MR do and why?

In !215778 (merged), a temporary solution was implemented to unblock frontend changes while backend work to implement this setting at the application and group level was in progress.

With all backend changes for enabling/disabling DAP now merged, duo_agent_platform_enabled will always be provided by the backend:

• Default value no longer needed
• showDuoAgentPlatformEnabledSetting removed, as this setting is now always displayed

References

Follow-up from "DAP enablement setting" (#583772)

Screenshots or screen recordings

Application-level	Group-level
Screen_Recording_2025-12-11_at_3.46.14_PM	🚧 TODO

How to set up and validate locally

Application-level setting:

• Run GDK on non-SASS mode: GITLAB_SIMULATE_SAAS=0 gdk start
• Go to /admin/gitlab_duo/configuration
• Note that GitLab Duo Agent Platform setting is displayed, toggling and saving persists the changes as expected.

Group-level setting:

• Run GDK on SASS mode: GITLAB_SIMULATE_SAAS=1 gdk start
• Go to /groups/gitlab_duo/-/settings/gitlab_duo/configuration
• Note that GitLab Duo Agent Platform setting is displayed, toggling and saving persists the changes as expected.

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