remove the deprecated register command (#13297)

# Description

This PR removes the `register` command which has been
[deprecated](https://www.nushell.sh/blog/2024-04-30-nushell_0_93_0.html#register-toc)
in favor of [`plugin
add`](https://www.nushell.sh/blog/2024-04-30-nushell_0_93_0.html#redesigned-plugin-management-commands-toc)

# User-Facing Changes

`register` is no longer available

crates/nu-cmd-plugin/src/commands/plugin/stop.rs

@@ -72,7 +72,7 @@ impl Command for PluginStop {
                 error: format!("Failed to stop the `{}` plugin", name.item),
                 msg: "couldn't find a plugin with this name".into(),
                 span: Some(name.span),
-                help: Some("you may need to `register` the plugin first".into()),
+                help: Some("you may need to `plugin add` the plugin first".into()),
                 inner: vec![],
             })
         }