Building `where_clause` in `UniquenessValidator` is no longer needed

Building `where_clause` manually was introduced at #26073 to include
both `comparison` and `binds` in `where_clause`. Since 213796fb,
`comparison` includes `binds`, so it is enough to use `where` simply.

activerecord/lib/active_record/validations/uniqueness.rb

@@ -79,10 +79,7 @@ def build_relation(klass, attribute, value)
         else
           klass.connection.case_sensitive_comparison(table, attribute, column, value)
         end
-        klass.unscoped.tap do |scope|
-          parts = [comparison]
-          scope.where_clause += Relation::WhereClause.new(parts)
-        end
+        klass.unscoped.where!(comparison)
       end
 
       def scope_relation(record, relation)