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/lib.rs

@@ -24,6 +24,3 @@ pub use parser::{
     is_math_expression_like, parse, parse_block, parse_expression, parse_external_call,
     parse_unit_value, trim_quotes, trim_quotes_str, unescape_unquote_string, DURATION_UNIT_GROUPS,
 };
-
-#[cfg(feature = "plugin")]
-pub use parse_keywords::parse_register;