Remove str deunicode (#13693)

# Description
Closes #13677

Remove the command `str deunicode`, as it has a narrow application, is
loosely defined by the data provided by the `deunicode` crate and thus a
stabilization liability post-1.0.

Furthermore the data to perform the look-up is quite substantial.

Removing the command and the `deunicode` dependency saves 0.9 MB of
binary data in release mode (~ 2% of total)

(checked via `cargo bloat --release` for a linux x86 build)


# User-Facing Changes
The `str deunicode` command recently added in #13270 is gone

crates/nu-command/src/default_context.rs

@@ -199,7 +199,6 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             Str,
             StrCapitalize,
             StrContains,
-            StrDeunicode,
             StrDistance,
             StrDowncase,
             StrEndswith,