Drop deprecated column from projects

What does this MR do?

As a cleanup task following #13251 (closed) and the deployment of !16187 (merged), we can drop the merge_requests_require_code_owner_approval column from the projects table.

== 20191015154408 DropMergeRequestsRequireCodeOwnerApprovalFromProjects: migrating
-- remove_column(:projects, :merge_requests_require_code_owner_approval, :boolean)
   -> 0.0017s
-- transaction_open?()
   -> 0.0000s
-- select_one("SELECT current_setting('server_version_num') AS v")
   -> 0.0005s
-- indexes(:projects)
   -> 0.0084s
== 20191015154408 DropMergeRequestsRequireCodeOwnerApprovalFromProjects: migrated (0.0115s)

Rollback procedure: This is a destructive migration, and can not be accurately rolled back. However, the data lost has been previously migrated off this table, and this data deprecated.

Closes #34636 (closed) First proposed in !18681 (closed)

Database checklist

• Conforms to the database guides

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body
• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)
• Added rollback procedure. Include either a rollback procedure or description how to rollback changes

When removing columns, tables, indexes or other structures:

• Removed these in a post-deployment migration
• Made sure the application no longer uses (or ignores) these structures