chore(pubkey): remove dead code (#18162)

(cherry picked from commit 755b7c7aee)

Co-authored-by: hrls <viktor.kharitonovich@gmail.com>

sdk/program/src/pubkey.rs

@@ -76,13 +76,11 @@ pub enum ParsePubkeyError {
     WrongSize,
     #[error("Invalid Base58 string")]
     Invalid,
-    #[error("Infallible")]
-    Infallible,
 }
 
 impl From<Infallible> for ParsePubkeyError {
     fn from(_: Infallible) -> Self {
-        Self::Infallible
+        unreachable!("Infallible unihnabited");
     }
 }