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-plugin/src/plugin/command.rs

@@ -340,7 +340,7 @@ where
 
 /// Build a [`PluginSignature`] from the signature-related methods on [`PluginCommand`].
 ///
-/// This is sent to the engine on `register`.
+/// This is sent to the engine on `plugin add`.
 ///
 /// This is not a public API.
 #[doc(hidden)]

crates/nu-plugin/src/plugin/mod.rs

@@ -38,7 +38,7 @@ pub(crate) const OUTPUT_BUFFER_SIZE: usize = 16384;
 /// The API for a Nushell plugin
 ///
 /// A plugin defines multiple commands, which are added to the engine when the user calls
-/// `register`.
+/// `plugin add`.
 ///
 /// The plugin must be able to be safely shared between threads, so that multiple invocations can
 /// be run in parallel. If interior mutability is desired, consider synchronization primitives such