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-parser/src/parser.rs

@@ -5275,15 +5275,6 @@ pub fn parse_expression(working_set: &mut StateWorkingSet, spans: &[Span]) -> Ex
             }
             b"where" => parse_where_expr(working_set, &spans[pos..]),
             #[cfg(feature = "plugin")]
-            b"register" => {
-                working_set.error(ParseError::BuiltinCommandInPipeline(
-                    "register".into(),
-                    spans[0],
-                ));
-
-                parse_call(working_set, &spans[pos..], spans[0])
-            }
-            #[cfg(feature = "plugin")]
             b"plugin" => {
                 if spans.len() > 1 && working_set.get_span_contents(spans[1]) == b"use" {
                     // only 'plugin use' is banned
@@ -5419,8 +5410,6 @@ pub fn parse_builtin_commands(
         b"export" => parse_export_in_block(working_set, lite_command),
         b"hide" => parse_hide(working_set, lite_command),
         b"where" => parse_where(working_set, lite_command),
-        #[cfg(feature = "plugin")]
-        b"register" => parse_register(working_set, lite_command),
         // Only "plugin use" is a keyword
         #[cfg(feature = "plugin")]
         b"plugin"