Remove addon-related lead creation logic from trials_controller.rb#create_addon

Summary

Remove the addon-related lead creation logic from trials_controller.rb#create_addon after successful migration to the new Leads::GitlabCom::AddonsController.

Problem

The trials_controller.rb#create_addon method currently handles both lead creation and addon trial application. After migration, it should only handle trial application.

Prerequisites

• New Leads::GitlabCom::AddonsController deployed and stable
• GitLab.com feature flag new_addon_lead_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#create_addon:

• Remove lead creation logic (keep addon trial application logic)
• Remove lead-specific validations
• Remove lead-specific helper methods
• Update method to only handle addon trial application

Important: Do NOT remove addon trial application logic - only the lead creation portion

Verification

Before removing code, verify:

• No traffic to legacy single-call flow in last 2 weeks (check logs/metrics)
• All GitLab.com instances using new two-step flow (lead → trial)
• No errors related to addon leads in production

Documentation Updates

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