🐛 Remove CollectionForm#alternative_title

[jeremyf]

The inline comments explain best:

As part of PR 87 we have attempted to rid the collection form of
the alternative_title (to deal with the conflicting predicate for
slugs). However, it is declared by way of
Hyrax::Forms::CollectionForm.delegate(:alternative_title, to: > :model)

Which means that we cannot rely on remove_method to tidy this up.
Instead we declare the method (thus obliterating the delegation).
We then use Ruby's undef_method which instructs the object to no
longer respond_to the undefined method. See the tests for
clarification.

Further, I've discovered that re-using a decorator that was declared in
Hyku resulted in the Knapsack decorator not working; hence the rename.

Related to:

• 🐛 Return identifier to collection show #87
• https://playbook-staging.notch8.com/en/samvera/custom-slugs
• Collection Show Page missing metadata fields (Knapsack) #251
• Admin Set show page is broken #291