Revert "Add subsciption and subscription_item to line_item PK (#28)"

This reverts commit 0d16f4e.

tap_stripe/__init__.py

@@ -25,10 +25,7 @@
     'invoices': {'sdk_object': stripe.Invoice, 'key_properties': ['id']},
     'invoice_items': {'sdk_object': stripe.InvoiceItem, 'key_properties': ['id']},
     'invoice_line_items': {'sdk_object': stripe.InvoiceLineItem,
-                           'key_properties': ['id',
-                                              'invoice',
-                                              'subscription',
-                                              'subscription_item']},
+                           'key_properties': ['id', 'invoice']},
     'transfers': {'sdk_object': stripe.Transfer, 'key_properties': ['id']},
     'coupons': {'sdk_object': stripe.Coupon, 'key_properties': ['id']},
     'subscriptions': {'sdk_object': stripe.Subscription, 'key_properties': ['id']},
@@ -590,14 +587,6 @@ def sync_sub_stream(sub_stream_name, parent_obj, updates=False):
             if sub_stream_name == "invoice_line_items":
                 # Synthetic addition of a key to the record we sync
                 obj_ad_dict["invoice"] = parent_obj.id
-
-                # Line item PKs must be a combination of all unique IDs associated
-                # with the different types a line item can be
-                # If they are not populated, they must be added here.
-                if "subscription" not in obj_ad_dict:
-                    obj_ad_dict["subscription"] = None
-                if "subscription_item" not in obj_ad_dict:
-                    obj_ad_dict["subscription_item"] = None
             elif sub_stream_name == "payout_transactions":
                 # payout_transactions is a join table
                 obj_ad_dict = {"id": obj_ad_dict['id'], "payout_id": parent_obj['id']}