Cleanup conversion of bigint ci build needs for self-managed

What does this MR do and why?

This MR removes columns and triggers used for the BigInt conversion of the ci_build_needs table for self-managed.

• removes column id_convert_to_bigint
• removes trigger_3207b8d0d6f3 and function

This has already been completed for SaaS: !115398 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

3/4 MR for: #415390 (closed)