Hide "Land to hosted git" button for now

Summary: Ref T182. This feature rarely/never works and is on the balance enormously confusing to users (see https://github.com/facebook/phabricator/issues/566). If installs have somehow made it sort of work, they can comment this line out for now until we have time to make this work more reasonably.

Test Plan: Looked at a revision in Differential.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D8719