Remove OsStrExt

[remram44]

Mentioned from #26730, this goes further and totally removes OsStrExt. I don't know if anyone will consider this but I happen to have written it.

GitHub shows previous PR's commits as well because this is how PRs work; this is actually one commit.

[rust-highfive]

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[alexcrichton]

The OsStrExt extension trait is a stable extension trait in Rust right now, so it cannot be removed without a deprecation path or some other similar means. Additionally it provides a crucial piece of functionality for each platform and is part of the core design of OsStr. The conversion functions that work across platforms aren't intended to be the only way of accessing data as they don't quite convey as much information as you'd need otherwise. As a result I'm going to close this, but thanks for the PR!