DEV: Cleanup unused wizard illustrations (#23659)

These were defunct since #19487

lib/wizard/builder.rb

@@ -10,7 +10,6 @@ class Wizard
       return @wizard unless SiteSetting.wizard_enabled? && @wizard.user.try(:staff?)
 
       @wizard.append_step("introduction") do |step|
-        step.banner = "welcome-illustration"
         step.emoji = "wave"
         step.description_vars = { base_path: Discourse.base_path }
 
@@ -59,7 +58,6 @@ class Wizard
       end
 
       @wizard.append_step("privacy") do |step|
-        step.banner = "members-illustration"
         step.emoji = "hugs"
         step.add_field(
           id: "login_required",
@@ -109,7 +107,6 @@ class Wizard
 
       @wizard.append_step("ready") do |step|
         # no form on this page, just info.
-        step.banner = "finished-illustration"
         step.emoji = "rocket"
       end