Clean up bigint conversion for ci_pipeline_variables.id

What does this MR do and why?

Clean up bigint conversion for ci_pipeline_variables.id

• drop column id_convert_to_bigint
• drop trigger trigger_023e82d8e257

This is the third step described in #408936 (closed)

Previous step is swapping the primary key using bigint, see !120823 (merged)

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Related to #408936 (closed)