Add `System.delete_env/1`

@josevalim I am sending this as a PR because I also changed a few places to use `String.to_char_list!` and `String.from_char_list!` instead of `iolist_to_binary` and `:binary.bin_to_list`. But I'm not sure if the changes are correct because a few other places in `system.ex` had the following comment:

```
# Notice we don't use unicode for conversion
# because the OS is expecting and returning raw bytes
```

Or if the current behaviour was because of performance reasons.