This is no longer needed with Rails 5.2. opclass is the attribute used
per https://github.com/rails/rails/pull/19090/files.

Now that we've removed the monkey patch and restored the Rails schema
dumper, it appears Rails has dropped the inclusion of `using: :btree` as
well (https://github.com/rails/rails/pull/27981).

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64529