Use multipart suggestion in unnecessary_to_owned (rust-lang#13847)

It is related to rust-lang/rust-clippy#13099

What was done:
- removed `@no-rustfix` from `tests/ui/unnecessary_to_owned.rs`
- checked for suggestions in `src/methods/unnecessary_to_owned.rs`
(there is already one multipart_suggestion, and I don't see any other)
- ran `cargo uibless`

I am new to Rust, and this is my first PR.

changelog: none

Author: Manishearth