Remove theme_option

This helper is no longer used.

crates/mdbook-html/src/html_handlebars/hbs_renderer.rs

@@ -271,8 +271,6 @@ impl HtmlHandlebars {
                 no_section_label: html_config.no_section_label,
             }),
         );
-        // TODO: remove theme_option in 0.5, it is not needed.
-        handlebars.register_helper("theme_option", Box::new(helpers::theme::theme_option));
     }
 
     fn emit_redirects(

crates/mdbook-html/src/html_handlebars/helpers/mod.rs

@@ -1,3 +1,2 @@
 pub(crate) mod resources;
-pub(crate) mod theme;
 pub(crate) mod toc;

crates/mdbook-html/src/html_handlebars/helpers/theme.rs

@@ -1,38 +0,0 @@
-use handlebars::{
-    Context, Handlebars, Helper, Output, RenderContext, RenderError, RenderErrorReason,
-};
-use log::trace;
-
-pub(crate) fn theme_option(
-    h: &Helper<'_>,
-    _r: &Handlebars<'_>,
-    ctx: &Context,
-    rc: &mut RenderContext<'_, '_>,
-    out: &mut dyn Output,
-) -> Result<(), RenderError> {
-    trace!("theme_option (handlebars helper)");
-
-    let param = h.param(0).and_then(|v| v.value().as_str()).ok_or_else(|| {
-        RenderErrorReason::ParamTypeMismatchForName(
-            "theme_option",
-            "0".to_owned(),
-            "string".to_owned(),
-        )
-    })?;
-
-    let default_theme = rc.evaluate(ctx, "@root/default_theme")?;
-    let default_theme_name = default_theme.as_json().as_str().ok_or_else(|| {
-        RenderErrorReason::ParamTypeMismatchForName(
-            "theme_option",
-            "default_theme".to_owned(),
-            "string".to_owned(),
-        )
-    })?;
-
-    out.write(param)?;
-    if param.to_lowercase() == default_theme_name.to_lowercase() {
-        out.write(" (default)")?;
-    }
-
-    Ok(())
-}