Remove code which overrides "diffusion.ssh-username" when instanced

Summary:
Ref T13529. Now that instances can be renamed, an instance may have multiple valid SSH usernames and the preferred SSH username may not be the intenal instance name.

PhacilitySiteSource should already always set diffusion.ssh-username correctly, to the current preferred SSH username (which may be "new-name" after a rename from "old-name"), so we should never be able to reach this code without an accurate diffusion.ssh-username value available.

The code to resolve names into instances also already works for both "ssh old-name@..." and "ssh new-name@...".

So I believe this code has no beneficial effects and only causes harm: it may force us to return "old-name" when falling through would correctly return "new-name".

Test Plan:

• Previously: renamed an instance, then SSH'd to it using both the old and new names. Both work.
• Previously: verified that diffusion.ssh-username is set correctly after a rename.
• Verified that Diffusion "Clone" UI now shows "new-name" after an instance rename.
• The real question here is: does this break something I'm not thinking of? And the change probably has to go to production to answer that.

Maniphest Tasks: T13529

Differential Revision: https://secure.phabricator.com/D21259