MB-100: Minor vtree_copy cleanup

Simplify the extract() function.

Change-Id: I58f9d6e7a6d5c52c9133ea75df64ce851ec976f5
Reviewed-on: http://review.couchbase.org/28075
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>