remove checksum

db/migrate/20190905152630_add_import_checksum_to_exercises.rb

@@ -1,5 +0,0 @@
-class AddImportChecksumToExercises < ActiveRecord::Migration[5.2]
-  def change
-    add_column :exercises, :import_checksum, :string
-  end
-end

db/schema.rb

@@ -153,7 +153,6 @@ ActiveRecord::Schema.define(version: 2019_10_08_163045) do
     t.boolean "allow_auto_completion", default: false
     t.integer "expected_difficulty", default: 1
     t.uuid "uuid"
-    t.string "import_checksum"
     t.boolean "unpublished", default: false
     t.index ["id"], name: "index_exercises_on_id"
   end