Resolve "Cleanup bigint conversion for merge_request_metrics"

What does this MR do and why?

This MR removes columns and triggers used for the bigint conversion of table merge_request_metrics.

• remove column id_convert_to_bigint
• remove trigger_c7107f30d69d trigger and function

Note: This merge request only affects self-host users.

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Related to #389516 (closed)