Eliminate dead code in nu-explore (#12735)

# Description
Nightly clippy found some unused fields leading me down a rabbit hole of
dead code hidden behind `pub`

Generally removing any already dead code or premature configurability
that is not exposed to the user.

# User-Facing Changes

None in effect.

Removed some options from the `$env.config.explore.hex-dump` record that
were only read into a struct but never used and also not validated.

crates/nu-explore/src/commands/mod.rs

@@ -51,10 +51,3 @@ pub trait ViewCommand {
         value: Option<Value>,
     ) -> Result<Self::View>;
 }
-
-#[derive(Debug, Default, Clone)]
-pub struct Shortcode {
-    pub code: &'static str,
-    pub context: &'static str,
-    pub description: &'static str,
-}