Remove ryu plugin

Ryu plugin was marked deprecated in Juno.
This commit actually removes the code for Kilo.

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

DocImpact
Partial-Bug: #1391714
Change-Id: I4916ce3c246730dc00516404471f8a1a008e27b6
Merge "Remove ryu plugin"