Remove dependency on IgnorableColumn concern

Summary

IgnorableColumn module in app/models/concerns/ignorable_column.rb which is used in models to ignore dealing with specified columns.

In Rails 5 with this PR , self.ignored_columns was introduced to do this job.

Example:

In app/models/user.rb

ignore_column :external_email
ignore_column :email_provider
ignore_column :authentication_token

can be changed to:

self.ignored_columns = %i[external_email email_provider authentication_token]

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32427