DEV: Remove register_color_scheme api (#28777)

This was added 10 years ago, but currently there's not a single use in our public and private plugins and no reference in third-party plugins on github

lib/plugin/instance.rb

@@ -49,7 +49,6 @@ class Plugin::Instance
   # Memoized array readers
   %i[
     assets
-    color_schemes
     initializers
     javascripts
     locales
@@ -593,12 +592,6 @@ class Plugin::Instance
   end
 
   def notify_after_initialize
-    color_schemes.each do |c|
-      unless ColorScheme.where(name: c[:name]).exists?
-        ColorScheme.create_from_base(name: c[:name], colors: c[:colors])
-      end
-    end
-
     initializers.each do |callback|
       begin
         callback.call(self)
@@ -732,10 +725,6 @@ class Plugin::Instance
     service_workers << [File.join(File.dirname(path), "assets", file), opts]
   end
 
-  def register_color_scheme(name, colors)
-    color_schemes << { name: name, colors: colors }
-  end
-
   def register_seed_data(key, value)
     seed_data[key] = value
   end