Remove unused cluster_providers_aws.created_by_user_id column

What does this MR do?

Removes an unused column - cluster_providers_aws.created_by_user_id (ignored in 13.3).

All rows have a null value for this column on GitLab.com.

Output

== 20200922231755 RemoveCreatedByUserIdFromClusterProvidersAws: reverting =====
-- column_exists?(:cluster_providers_aws, :created_by_user_id)
   -> 0.0026s
-- add_column(:cluster_providers_aws, :created_by_user_id, :integer)
   -> 0.0012s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:cluster_providers_aws, :created_by_user_id, {:algorithm=>:concurrently})
   -> 0.0025s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- add_index(:cluster_providers_aws, :created_by_user_id, {:algorithm=>:concurrently})
   -> 0.0027s
-- execute("RESET ALL")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:cluster_providers_aws)
   -> 0.0025s
-- execute("ALTER TABLE cluster_providers_aws\nADD CONSTRAINT fk_ed1fdfaeb2\nFOREIGN KEY (created_by_user_id)\nREFERENCES users (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0014s
-- execute("ALTER TABLE cluster_providers_aws VALIDATE CONSTRAINT fk_ed1fdfaeb2;")
   -> 0.0023s
== 20200922231755 RemoveCreatedByUserIdFromClusterProvidersAws: reverted (0.0198s)

== 20200922231755 RemoveCreatedByUserIdFromClusterProvidersAws: migrating =====
-- remove_column(:cluster_providers_aws, :created_by_user_id)
   -> 0.0027s
== 20200922231755 RemoveCreatedByUserIdFromClusterProvidersAws: migrated (0.0115s)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#35223 (closed)