R1:b1351d0fdb81

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…