Cleanup bigint conversion for deployments

What does this MR do?

What does this MR do?

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

• remove column deployable_id_convert_to_bigint
• remove trigger_77f5e1d20482 trigger and function

Related #338054 (closed).

Database migration

Up

$ bundle exec rails db:migrate:up VERSION=20210907033745
== 20210907033745 CleanupBigintConversionForDeployments: migrating ============
-- remove_column(:deployments, "deployable_id_convert_to_bigint")
   -> 0.0012s
== 20210907033745 CleanupBigintConversionForDeployments: migrated (0.0218s) ===

Down:

$ bundle exec rails db:migrate:down VERSION=20210907033745
== 20210907033745 CleanupBigintConversionForDeployments: reverting ============
-- table_exists?(:deployments)
   -> 0.0006s
-- column_exists?(:deployments, :id)
   -> 0.0013s
-- column_exists?(:deployments, :deployable_id)
   -> 0.0014s
-- columns(:deployments)
   -> 0.0014s
-- add_column(:deployments, "deployable_id_convert_to_bigint", :int, {:default=>nil})
   -> 0.0025s
== 20210907033745 CleanupBigintConversionForDeployments: reverted (0.0377s) ===

Does this MR meet the acceptance criteria?

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• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• Label as security and @ mention @gitlab-com/gl-security/appsec
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Related to #338054 (closed)