Delete vtree_copy module

The vtree_copy module is no longer used, the initial index build is
now done in couchstore.

Change-Id: I4d79df6680b94ea07d259a58da4ae805f3267708
Reviewed-on: http://review.couchbase.org/41950
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>