Remove usage of deprecated constant IRB::InputCompletor

Debug seems to be using a deprecated class from IRB: 

```
warning: constant IRB::InputCompletor is deprecated
```

`IRB::InputCompletor` was removed in a refactor: https://github.com/ruby/irb/pull/707, and then restored and deprecated in https://github.com/ruby/irb/commit/77265efc5f179d31be4ddea5d8fd6a5290fe678a