Remove unused column

The index in schema.rb is not directly related to this modification, it should
have been part of 2b763e4484 but was presumably
forgotten

db/migrate/20190311140926_remove_type_from_types_de_champ.rb

@@ -0,0 +1,5 @@
+class RemoveTypeFromTypesDeChamp < ActiveRecord::Migration[5.2]
+  def change
+    remove_column :types_de_champ, :type, :string
+  end
+end