Remove path.getowners method

Summary: This has been deprecated for quite a while and I'm pretty sure there are no callsites in the wild since this tool doesn't get much use outside of Facebook.

Test Plan: grep

Reviewers: vrana, btrahan, meitros

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3195