Remove unused error return

The `to_public` method on `DescriptorSinglePriv` never errors, no need
to return an error type.

Author: tcharding

src/descriptor/key.rs

@@ -138,17 +138,14 @@ pub struct DescriptorXKey<K: InnerXKey> {
 }
 
 impl DescriptorSinglePriv {
-    /// Returns the public key of this key
-    fn to_public<C: Signing>(
-        &self,
-        secp: &Secp256k1<C>,
-    ) -> Result<DescriptorSinglePub, DescriptorKeyParseError> {
+    /// Returns the public key of this key.
+    fn to_public<C: Signing>(&self, secp: &Secp256k1<C>) -> DescriptorSinglePub {
         let pub_key = self.key.public_key(secp);
 
-        Ok(DescriptorSinglePub {
+        DescriptorSinglePub {
             origin: self.origin.clone(),
             key: SinglePubKey::FullKey(pub_key),
-        })
+        }
     }
 }