Fix the unused variable warning

This fixes the following warning.

```ruby
warning: assigned but unused variable - index_definition
```

activerecord/test/cases/invertible_migration_test.rb

@@ -241,7 +241,6 @@ def test_migrate_nested_revert_whole_migration
     end
 
     def test_migrate_revert_change_column_default
-      index_definition = ["horses", [:name, :color]]
       migration1 = ChangeColumnDefault1.new
       migration1.migrate(:up)
       assert_equal "Sekitoba", Horse.new.name