Remove legacy Ultimate trial code path from trials_controller.rb

Summary

Remove the legacy Ultimate trial application code from trials_controller.rb after successful migration to the new Trials::GitlabCom::UltimatesController.

Prerequisites

• New Trials::GitlabCom::UltimatesController deployed and stable
• GitLab.com feature flag new_ultimate_trial_endpoint rolled out to 100%
• Feature flag has been at 100% for at least 2 weeks with no issues
• GitLab.com legacy routing code removed

Implementation Details

Code Removal

From trials_controller.rb:

• Remove Ultimate trial application logic from create method
• Remove Ultimate-specific before_actions (if any)
• Remove Ultimate-specific validations
• Remove Ultimate-specific helper methods
• Update routes to remove legacy Ultimate trial endpoint

Verification

Before removing code, verify:

• No traffic to legacy endpoint in last 2 weeks (check logs/metrics)
• All GitLab.com instances using new endpoint
• No errors related to Ultimate trials in production

Documentation Updates

• Add new documentation of this flow in the CDOT trial flow docs