The `"nice weather".into()` line could be solved with either
`string_slice` or `string`. The `string_slice` choice compiled and
passed the exercise while emitting a `clippy::useless_conversion`
warning, so the exercise could pass with a warning still present.
Understanding why `.into()` works for both targets requires the `From`
trait, which rustlings introduces later. Remove the example from the
exercise and solution.
Fixes#2190