Remove hkdf::Salt::derive.

A new API for it will be provided later.

src/hkdf.rs

@@ -38,16 +38,6 @@ use crate::{digest, error, hmac};
 pub struct Salt(hmac::Key);
 
 impl Salt {
-    /// The [HKDF-Expand] operation, specialized for deriving a new salt.
-    ///
-    /// The new `Salt` will have a length equal to the digest algorithm's output
-    /// length.
-    ///
-    /// [HKDF-Expand]: https://tools.ietf.org/html/rfc5869#section-2.3
-    pub fn derive(digest_algorithm: &'static digest::Algorithm, okm: Okm) -> Self {
-        Self(hmac::Key::derive(digest_algorithm, okm))
-    }
-
     /// Constructs a new `Salt` with the given value based on the given digest
     /// algorithm.
     ///