sheep: cleaning vdi deletion process, round 2

This patch cleans the vdi deletion process of sheep. Current vdi
deletion code creates one work for one deletion. It seems that there
is no reason to create so many work. This patch lets the code to
create only one work for vdi deletion request.

With this change, this patch improves simplicity of the code. For
example, the queue of struct deletion_info and related code are
removed.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>