Remove deprecated behaviour of `#table_exists?`

[c960657]

Since Rails 5.1 #tables only returns tables, not views.

• https://github.com/rails/rails/blob/5-1-stable/activerecord/CHANGELOG.md#rails-510-april-27-2017
• https://github.com/rails/rails/commit/5973a984c369a63720c2ac18b71012b8347479a8

Together with #25 this PR removes all deprecated usage of ActiveSupport::Deprecation.warn (see https://github.com/rails/rails/pull/47354).