Address a failure test_remove_column_with_multi_column_index with Oracle

This pull request addresses the following failure.

```ruby
$ ARCONN=oracle ruby -Itest test/cases/migration/rename_column_test.rb -n test_remove_column_with_multi_column_index
Using oracle
Run options: -n test_remove_column_with_multi_column_index --seed 24358

# Running tests:

F

Finished tests in 4.192528s, 0.2385 tests/s, 0.4770 assertions/s.

  1) Failure:
test_remove_column_with_multi_column_index(ActiveRecord::Migration::RenameColumnTest) [test/cases/migration/rename_column_test.rb:114]:
--- expected
+++ actual
@@ -1 +1 @@
-["index_test_models_on_hat_style_and_hat_size"]
+[]


1 tests, 2 assertions, 1 failures, 0 errors, 0 skips
$
```

Not only PostgreSQL, Oracle database adapter drops the multi-column index if any of the indexed columns are dropped by remove_column.