[EE] Remove PostgreSQL opclasses monkey patch
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 in the schema (https://github.com/rails/rails/pull/27981).
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59673
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64529
EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30709.
Edited by Stan Hu