Remove legacy addon trial code path from trials_controller.rb

Summary

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

Problem

After migration, trials_controller.rb#create_addon should be completely removed as all addon trial logic has been extracted.

Prerequisites

• New Trials::GitlabCom::AddonsController deployed and stable
• GitLab.com feature flag new_addon_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 create_addon method entirely
• Remove addon-specific before_actions (if any)
• Remove addon-specific validations
• Remove addon-specific helper methods
• Update routes to remove legacy addon 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 addon trials in production

Documentation Updates

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