refactor(smtp): remove unused SendTimeout Error variant

It was used when timeout was set on the whole smtp.send() operation.
Now only the operations inside smtp.send() can timeout, and such timeout
errors result in SendError, so SendTimeout is unused.

src/smtp/send.rs

@@ -18,15 +18,6 @@ pub enum Error {
 
     #[fail(display = "SMTP has no transport")]
     NoTransport,
-
-    #[fail(display = "SMTP send timed out")]
-    SendTimeout(#[cause] async_std::future::TimeoutError),
-}
-
-impl From<async_std::future::TimeoutError> for Error {
-    fn from(err: async_std::future::TimeoutError) -> Error {
-        Error::SendTimeout(err)
-    }
 }
 
 impl Smtp {