docs: Removing IAB v2 API docs

In-App Billing Version 2 has been deprecated for years, and was
finally removed in January 2015. Removing its API docs and references
to those docs; there's a second CL (http://ag/782481 for redirects from
those pages to the v3 docs.

See first comment for doc stage location.

bug: 19169884
Change-Id: Ie38e7075779754ef8a2310b0b2bd8b276a781eca