Remove legacy iterable lead code path from trials_controller.rb

Summary

Remove the legacy iterable lead creation code from trials_controller.rb after successful migration to the new Leads::GitlabCom::IterablesController.

Prerequisites

• New Leads::GitlabCom::IterablesController deployed and stable (#15117)
• GitLab.com feature flag new_iterable_lead_endpoint rolled out to 100% (gitlab#582577)
• 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 create_iterable method
• Remove iterable-specific before_actions (if any)
• Remove iterable-specific validations
• Remove iterable-specific helper methods
• Update routes to remove legacy iterable 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 iterable leads in production

Testing

• Remove tests specific to legacy iterable code path
• Ensure remaining trials_controller.rb tests still pass
• Verify new controller tests are comprehensive

Acceptance Criteria

• Legacy iterable code removed from trials_controller.rb
• Legacy routes removed
• Legacy tests removed
• All remaining tests pass
• Code review completed
• Deployed to production
• Verified no errors in production for 1 week post-deployment

Documentation Updates

• Add new documentation of this flow in the CDOT docs. See trial flow as an example.