docs: Noted that orderId is blank for all test purchases.

All test purchases associated with Google Play apps---both one-time
and subscription---now contain a blank "orderId" field. Updated the
documentation accordingly. Also removed the explanation about
canceling test purchases using the merchant center; a lack of an
"orderId" value prevents this workflow from being possible anymore.

Bug: 29068371
Change-Id: I748989c404549769eefd44afa3a7cc125f2e7073