FIX: Hide core plugins from the admin Plugins list (#23328)

Most of the core plugins were already hidden, this hides
chat, styleguide, and checklist to avoid potential confusion
for end users.

Also removes respond_to? :hide_plugin, since that API has been
in place for a while now.

plugins/checklist/plugin.rb

@@ -10,3 +10,4 @@ enabled_site_setting :checklist_enabled
 
 register_asset "stylesheets/checklist.scss"
 register_svg_icon "spinner"
+hide_plugin