Rollup merge of rust-lang#62224 - euclio:remove-derives, r=GuillaumeGomez

rustdoc: remove unused derives and variants

Though many structs in rustdoc derive `RustcEncodable` and `RustcDecodable`, the impls do not appear to be used by the crate or its dependents. Removing them revealed some enum variants that are never constructed, too.

r? @GuillaumeGomez

Author: Centril

src/librustdoc/clean/cfg.rs

@@ -16,7 +16,7 @@ use syntax_pos::Span;
 
 use crate::html::escape::Escape;
 
-#[derive(Clone, RustcEncodable, RustcDecodable, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub enum Cfg {
     /// Accepts all configurations.
     True,