Cleanup bigint conversion for ci_stages

What does this MR do?

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

• remove column id_convert_to_bigint
• remove trigger_490d204c00b3 trigger and function

Related to #338066 (closed).

Database migration

Up

$ bundle exec rails db:migrate:up VERSION=20210907021940
== 20210907021940 CleanupBigintConversionForCiStages: migrating ===============
-- remove_column(:ci_stages, "id_convert_to_bigint")
   -> 0.0026s
== 20210907021940 CleanupBigintConversionForCiStages: migrated (0.0184s) ======

Down

$ bundle exec rails db:migrate:down VERSION=20210907021940
== 20210907021940 CleanupBigintConversionForCiStages: reverting ===============
-- table_exists?(:ci_stages)
   -> 0.0004s
-- column_exists?(:ci_stages, :id)
   -> 0.0011s
-- column_exists?(:ci_stages, :id)
   -> 0.0010s
-- columns(:ci_stages)
   -> 0.0010s
-- add_column(:ci_stages, "id_convert_to_bigint", :int, {:default=>0, :null=>false})
   -> 0.0037s
== 20210907021940 CleanupBigintConversionForCiStages: reverted (0.0341s) ======

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
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Related to #338066 (closed)