Remove updated_at column from audit_events table

What does this MR do?

Remove updated_at column from audit_events table.

Followed development guideline on dropping column.

• Step 1: Ignoring the column (release 13.1) (!34156 (merged))
• Step 2: Dropping the column (release 13.2) (this MR)

Relates to #217941 (closed)

Database log

== 20200724100421 RemoveUpdatedAtFromAuditEvents: reverting ===================
-- add_column("audit_events", :updated_at, :datetime)
   -> 0.0012s
-- add_column("audit_events_part_5fc467ac26", :updated_at, :datetime)
   -> 0.0016s
-- execute("CREATE OR REPLACE FUNCTION table_sync_function_2be879775d()\nRETURNS TRIGGER AS\n$$\nBEGIN\nIF (TG_OP = 'DELETE') THEN\n  DELETE FROM audit_events_part_5fc467ac26 where id = OLD.id;\nELSIF (TG_OP = 'UPDATE') THEN\n  UPDATE audit_events_part_5fc467ac26\n  SET author_id = NEW.author_id,\n    type = NEW.type,\n    entity_id = NEW.entity_id,\n    entity_type = NEW.entity_type,\n    details = NEW.details,\n    updated_at = NEW.updated_at,\n    ip_address = NEW.ip_address,\n    author_name = NEW.author_name,\n    entity_path = NEW.entity_path,\n    target_details = NEW.target_details,\n    created_at = NEW.created_at\n  WHERE audit_events_part_5fc467ac26.id = NEW.id;\nELSIF (TG_OP = 'INSERT') THEN\n  INSERT INTO audit_events_part_5fc467ac26 (id,\n    author_id,\n    type,\n    entity_id,\n    entity_type,\n    details,\n    updated_at,\n    ip_address,\n    author_name,\n    entity_path,\n    target_details,\n    created_at)\n  VALUES (NEW.id,\n    NEW.author_id,\n    NEW.type,\n    NEW.entity_id,\n    NEW.entity_type,\n    NEW.details,\n    NEW.updated_at,\n    NEW.ip_address,\n    NEW.author_name,\n    NEW.entity_path,\n    NEW.target_details,\n    NEW.created_at);\nEND IF;\nRETURN NULL;\n\nEND\n$$ LANGUAGE PLPGSQL\n")
   -> 0.0022s
== 20200724100421 RemoveUpdatedAtFromAuditEvents: reverted (0.0120s) ==========

== 20200724100421 RemoveUpdatedAtFromAuditEvents: migrating ===================
-- remove_column("audit_events", :updated_at)
   -> 0.0011s
-- execute("CREATE OR REPLACE FUNCTION table_sync_function_2be879775d()\nRETURNS TRIGGER AS\n$$\nBEGIN\nIF (TG_OP = 'DELETE') THEN\n  DELETE FROM audit_events_part_5fc467ac26 where id = OLD.id;\nELSIF (TG_OP = 'UPDATE') THEN\n  UPDATE audit_events_part_5fc467ac26\n  SET author_id = NEW.author_id,\n    type = NEW.type,\n    entity_id = NEW.entity_id,\n    entity_type = NEW.entity_type,\n    details = NEW.details,\n    ip_address = NEW.ip_address,\n    author_name = NEW.author_name,\n    entity_path = NEW.entity_path,\n    target_details = NEW.target_details,\n    created_at = NEW.created_at\n  WHERE audit_events_part_5fc467ac26.id = NEW.id;\nELSIF (TG_OP = 'INSERT') THEN\n  INSERT INTO audit_events_part_5fc467ac26 (id,\n    author_id,\n    type,\n    entity_id,\n    entity_type,\n    details,\n    ip_address,\n    author_name,\n    entity_path,\n    target_details,\n    created_at)\n  VALUES (NEW.id,\n    NEW.author_id,\n    NEW.type,\n    NEW.entity_id,\n    NEW.entity_type,\n    NEW.details,\n    NEW.ip_address,\n    NEW.author_name,\n    NEW.entity_path,\n    NEW.target_details,\n    NEW.created_at);\nEND IF;\nRETURN NULL;\n\nEND\n$$ LANGUAGE PLPGSQL\n")
   -> 0.0027s
-- remove_column("audit_events_part_5fc467ac26", :updated_at)
   -> 0.0030s
== 20200724100421 RemoveUpdatedAtFromAuditEvents: migrated (0.0101s) ==========

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