Remove unused AV helper fixtures from e10a2531

Several fixtures for helpers are removed. They were introduced in
ActionView by @strzalek but never referenced in any test.
Merge pull request #12232 from claudiob/delete-unused-helper-fixtures

Remove unused AV helper fixtures from e10a2531
ActiveRecord::ConnectionAdapters::Column.string_to_time method respects string with timezone. Closes #12278.