client: Remove Result from Mechanism::initial().

sasl/src/client/mod.rs

@@ -11,8 +11,8 @@ pub trait Mechanism {
         Self: Sized;
 
     /// Provides initial payload of the SASL mechanism.
-    fn initial(&mut self) -> Result<Vec<u8>, String> {
-        Ok(Vec::new())
+    fn initial(&mut self) -> Vec<u8> {
+        Vec::new()
     }
 
     /// Creates a response to the SASL challenge.