After the PR #39814 being merged, we can access the `[[algorithm]]`
internal slot of a `CryptoKey` directly in Rust, without doing a JS
object conversion.
This patch leverages this feature to simplify HMAC operation steps that
access the `[[algorithm]]` internal slot of `CryptoKey`.
Testing: Refactoring. Existing WPT tests suffice.
Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>