Drop source_id and source_type columns from spree_adjustments

These are no longer used

db/migrate/20211029174211_drop_source_columns.rb

@@ -0,0 +1,6 @@
+class DropSourceColumns < ActiveRecord::Migration[6.1]
+  def change
+    remove_column :spree_adjustments, :source_id
+    remove_column :spree_adjustments, :source_type
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_09_27_091723) do
+ActiveRecord::Schema.define(version: 2021_10_29_174211) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -373,10 +373,8 @@ ActiveRecord::Schema.define(version: 2021_09_27_091723) do
   end
 
   create_table "spree_adjustments", force: :cascade do |t|
-    t.integer "source_id"
     t.decimal "amount", precision: 10, scale: 2
     t.string "label", limit: 255
-    t.string "source_type", limit: 255
     t.integer "adjustable_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false