Remove old "Landing Strategy" code

Summary:
Fixes T12869. This is a very old, pre-Drydock chunk of code from D7486 and some followups.

It does three things:

• "Land to Hosted Git": Obsoleted by Drydock, has been commented out in HEAD for a very long time with no complaints. Disabled by D8719 in 2014.
• "Land to Hosted Mercurial": Could be obsoleted by Drydock with a fairly small amount of work, but currently has no replacement. Unclear if this sees any real use. Not actually disabled at HEAD.
• "Land to GitHub": Use GitHub OAuth credentials to land to GitHub. This is sort of theoretically useful and has no analog today. Disabled by D13022 in 2015.

This stuff was largely disabled a long time ago and we haven't seen users hitting issues with it. This could all be moved to an extension today if anyone still relies on it.

Test Plan: Grepped for removed classes, browsed Differential.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12869

Differential Revision: https://secure.phabricator.com/D18150