Simplify method to get existing locales

We can get the same results using `pluck`.

app/models/i18n_content_translation.rb

@@ -1,5 +1,5 @@
 class I18nContentTranslation < ApplicationRecord
   def self.existing_locales
-    self.select(:locale).distinct.map { |l| l.locale.to_sym }.to_a
+    distinct.pluck(:locale).map(&:to_sym)
   end
 end