Tidy up BackTraceFilter

### 🤔 What's the problem you've observed?

There's a lot of code here untouched in a decade. Some of it likely can be tidied up

### ✨ Do you have a proposal for making it better?

Remove some conditionals, check to see which Env vars are used and in what context

### 📚 Any additional context?

Found whilst fixing up this file for ruby 3.4