Remove the entry for obsolete ryu plugin (check-tempest-dsvm-ryuplugin)

Ryu plugin was marked deprecated in Juno and will be removed for Kilo.

We (Ryu team) recommend users to migrate to ofagent, on which
we aim to concentrate our development resources by this deprecation.

Partial-Bug: #1391714
Change-Id: If5cf02915f63790ed1b08a03e52701f4108643e0
Merge "Remove the entry for obsolete ryu plugin (check-tempest-dsvm-ryuplugin)"