Refactor: RaftError: serde derive

The problem is not the bound on the `E` generic, but rather, the fact
that `NID` is receiving two different `Serialize/Deserialize` bounds;
one from the derive annotation, and one from the `NID: NodeId` trait
bound.

This modification is from suggestions by @tvsfs about the serde bound:
#991 (comment)

openraft/src/error.rs

@@ -24,18 +24,14 @@ use crate::Vote;
 /// It is either a Fatal error indicating that `Raft` is no longer running, such as underlying IO
 /// error, or an API error `E`.
 #[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
-#[cfg_attr(
-    feature = "serde",
-    derive(serde::Deserialize, serde::Serialize),
-    serde(bound(serialize = "E: serde::Serialize")),
-    serde(bound(deserialize = "E: for <'d> serde::Deserialize<'d>"))
-)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub enum RaftError<NID, E = Infallible>
 where NID: NodeId
 {
     #[error(transparent)]
     APIError(E),
 
+    #[serde(bound = "")]
     #[error(transparent)]
     Fatal(#[from] Fatal<NID>),
 }