Remove theme_option

This helper is no longer used.

Author: ehuss

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;