rm regex

```rust
static RE: once_cell::sync::Lazy =
once_cell::sync::Lazy::new(|| regex::Regex::new(r#"\$ENV\{([\w][\w|\d|\.|_]*)\}"#).unwrap());
```

Didn't this get changed in log4rs? Or rather, wasn't the regex stuff removed? If so maybe we should consider removing it here as well, as a part of future work.

Read the static RE declaration shown in the issue and compare the related change in log4rs. Identify whether the regex machinery is still needed; done means the obsolete regex-related code is removed without breaking trace4rs.

Written by the indexing model from the issue text.