Make `app/models/clusters/concerns/application_data.rb` a plain module

Considering that https://gitlab.com/gitlab-org/gitlab-ce/blob/8c10e99de29f3c6f0d17ebde5af14324956dc109/app/models/clusters/concerns/application_data.rb is all inside of included then we do not need it to actually extend ActiveSupport::Concern.

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27348#note_160725738