DEV: Remove 'diff local changes' when updating remote themes (#11247)

Since 65e123498b, it is now impossible to make local changes to remote themes, so this warning is not needed.

app/models/remote_theme.rb

@@ -200,21 +200,6 @@ class RemoteTheme < ActiveRecord::Base
     end
   end
 
-  def diff_local_changes
-    return unless is_git?
-    importer = ThemeStore::GitImporter.new(remote_url, private_key: private_key, branch: branch)
-    begin
-      importer.import!
-    rescue RemoteTheme::ImportError => err
-      { error: err.message }
-    else
-      changes = importer.diff_local_changes(self.id)
-      return nil if changes.blank?
-
-      { diff: changes }
-    end
-  end
-
   def normalize_override(hex)
     return unless hex