FEATURE: Remove "Enable Sidebar" step from setup wizard (#26926)

This keeps coming up in user testing as something
we want to get rid of. The `navigation_menu` setting
has been set to sidebar by default for some time now,
and we are rolling out admin sidebar widely. It just
doesn't make sense to let people turn this off in
the first step of the wizard -- we _want_ people to
use the sidebar.

lib/wizard/builder.rb

@@ -89,13 +89,6 @@ class Wizard
           )
         end
 
-        step.add_field(
-          id: "enable_sidebar",
-          type: "checkbox",
-          icon: "bars",
-          value: SiteSetting.navigation_menu == NavigationMenuSiteSetting::SIDEBAR,
-        )
-
         step.on_update do |updater|
           updater.update_setting(:login_required, updater.fields[:login_required])
           updater.update_setting(:invite_only, updater.fields[:invite_only])