Remove some old code handling the absence of the file.uploadhash Conduit method

Summary: The file.uploadhash method was added a long time ago (in D4899). It should be safe to assume that this method exists on most installs.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11118