Resolve "Cleanup bigint conversion for merge_request_metrics"

What does this MR do?

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

bundle exec rake db:migrate:up:main VERSION=20230322151635

main: == 20230322151635 CleanupBigintConversionForMergeRequestMetrics: migrating ====
main: -- transaction_open?()
main:    -> 0.0000s
main: -- remove_column(:merge_request_metrics, "id_convert_to_bigint")
main:    -> 0.0012s
main: == 20230322151635 CleanupBigintConversionForMergeRequestMetrics: migrated (0.0296s)

bundle exec rake db:migrate:down:main VERSION=20230322151635

main: == 20230322151635 CleanupBigintConversionForMergeRequestMetrics: reverting ====
main: -- table_exists?(:merge_request_metrics)
main:    -> 0.0662s
main: -- column_exists?(:merge_request_metrics, :id)
main:    -> 0.0029s
main: -- column_exists?(:merge_request_metrics, :id)
main:    -> 0.0014s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- columns(:merge_request_metrics)
main:    -> 0.0016s
main: -- add_column(:merge_request_metrics, "id_convert_to_bigint", :int, {:default=>0, :null=>false})
main:    -> 0.0021s
main: == 20230322151635 CleanupBigintConversionForMergeRequestMetrics: reverted (0.0990s)

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Related to #389516 (closed)