Remove deprecated attributes from Debates

Some fields from Debates are deprecated and we no longer need them.

db/migrate/20200831112936_remove_deprecated_fields_from_debates.rb

@@ -0,0 +1,6 @@
+class RemoveDeprecatedFieldsFromDebates < ActiveRecord::Migration[5.1]
+  def change
+    remove_column :debates, :deprecated_title, :string
+    remove_column :debates, :deprecated_description, :text
+  end
+end