Drop greenio dependency

greenio dependency was added to add a new "greenio executor" to Oslo
Messaging, but I abandonned the changeset:
https://review.openstack.org/#/c/108652/

No OpenStack component use greenio.

Change-Id: I6a97e3b462f8d0985532384ea169ebee4c9b3c2d
Merge "Drop greenio dependency"