Remove git::clone support for converting clones to shared

The back story on this feature is that Ori and I were trying to get
puppet to manage a clone of mediawiki/tools/scap that had initially been
cloned as root only and needed to be updateable by a non-root (me). Scap
has since be changed to deploy via Trebuchet so I think at this point
the labs_vagrant role is the only user of the shared clone logic.

The puppet code that attempted to convert an existing clone from
non-shared to shared was prone to causing Puppet log spam and
potentially changing the permissions on files that were not part of the
actual git repository.

Change-Id: I86883ec708b104c8d5d267672b4c1a24453ea21b
Merge "Remove git::clone support for converting clones to shared" into production