Remove setWorkingDirectory call on SFTP interface

Summary: I derped on this; the SFTP interface doesn't have setWorkingDirectory because it implements DrydockFilesystemInterface and not DrydockCommandInterface.  So when you use the Upload File build step, the daemon will crash due to an undefined method.

Test Plan: Tested on my live server.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10351