Remove triggers and old columns for ci_pipelines loose FKs

Problem

This column references the ci_pipelines table and its column type is not bigint.

Process

This is part of the process to convert the column to bigint.

Follow the instruction, and remove the triggers and integer columns for:

• merge_requests.head_pipeline_id_convert_to_bigint
• merge_request_metrics.pipeline_id_convert_to_bigint
• packages_build_infos.pipeline_id_convert_to_bigint
• merge_trains.pipeline_id_convert_to_bigint
• vulnerability_feedback.pipeline_id_convert_to_bigint
• vulnerability_occurrence_pipelines.pipeline_id_convert_to_bigint