Revert passing arel node with splat binds for where

Passing arel node with splat binds for `where` was introduced at #22877
for uniqueness validator supports prepared statement. But I'd not like
to introduce the following usage:

```ruby
  Foo.where(arel, *binds)
```

I'd like to revert this internal usage.

activerecord/lib/active_record/relation/where_clause_factory.rb

@@ -22,7 +22,6 @@ module ActiveRecord
           parts = predicate_builder.build_from_hash(attributes)
         when Arel::Nodes::Node
           parts = [opts]
-          binds = other
         else
           raise ArgumentError, "Unsupported argument type: #{opts} (#{opts.class})"
         end

activerecord/lib/active_record/validations/uniqueness.rb

@@ -76,8 +76,11 @@ module ActiveRecord
         else
           klass.connection.case_sensitive_comparison(table, attribute, column, value)
         end
-        bind = Relation::QueryAttribute.new(attribute_name, value, cast_type)
-        klass.unscoped.where!(comparison, bind)
+        klass.unscoped.tap do |scope|
+          parts = [comparison]
+          binds = [Relation::QueryAttribute.new(attribute_name, value, cast_type)]
+          scope.where_clause += Relation::WhereClause.new(parts, binds)
+        end
       end
 
       def scope_relation(record, relation)