Apply suggestions from code review

Co-authored-by: Stephane Raux <[email protected]>

mls-rs/src/group/external_commit.rs

@@ -124,7 +124,7 @@ impl<C: ClientConfig> ExternalCommitBuilder<C> {
         self
     }
 
-    #[cfg(feature = "custom_proposal")]
+    #[cfg(all(feature = "custom_proposal", feature = "by_ref_proposal"))]
     #[must_use]
     /// Insert a [`CustomProposal`] received from a current group member into the current
     /// commit that is being built.
@@ -135,7 +135,6 @@ impl<C: ClientConfig> ExternalCommitBuilder<C> {
     /// same way as by [`ExternalGroup`](`crate::external_client::ExternalGroup`).
     /// The proposal MUST be an MlsPlaintext, else the [`Self::build`] function will fail.
     pub fn with_received_custom_proposal(mut self, proposal: MlsMessage) -> Self {
-        //use super::message_verifier::verify_plaintext_authentication;
 
         self.received_custom_proposals.push(proposal);
         self