Remove very old "owners-default-path" code from Owners

Summary: Ref T12590. This is ancient code which was used to prefill /trunk/tfb/www/ or similar at Facebook. I don't think it ever had a UI and no install has asked for this feature since 2011.

Test Plan: Grepped for affected symbols, edited paths in Owners.

Maniphest Tasks: T12590

Differential Revision: https://secure.phabricator.com/D19189