proto: remove incorrect feature guard

`HashedConnectionIdGenerator::new()` uses the mandatory rand crate to
generate a key by default, not the *ring* crate.

quinn-proto/src/cid_generator.rs

@@ -125,7 +125,6 @@ impl HashedConnectionIdGenerator {
     }
 }
 
-#[cfg(feature = "ring")]
 impl Default for HashedConnectionIdGenerator {
     fn default() -> Self {
         Self::new()