Fix that a1bb6c8b06db8354617 overloaded the uniq-like behavior of CollectionAssociation#uniq with #distinct

Everywhere else in ActiveRecord there's a clear association between
the #distinct method and the DISTINCT sql statement.
CollectionAssociation#uniq was not at all scope-like and rather returns
an array, as #uniq does elsewhere in Ruby.

IMO overloading this behavior with #distinct's other behavior will cause
confusion - better to provide these 2 behaviors separately.