Remove ignore_column for ci_pipeline_variables.id_convert_to_bigint

What does this MR do and why?

As titled, remove ignore_column for ci_pipeline_variables.id_convert_to_bigint since the column id is now bigint and the column id_convert_to_bigint is removed in %16.2.

See step 4 of Migrating integer primary keys to bigint

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Related to #408936 (closed)