Remove ryu plugin

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

Also, mention Ryu team's recommendation for ofagent in
doc/config-reference/networking/section_networking-plugins.xml

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: Ia11ce42f1cd3ad9d1ccc2e7bf2c2937d7ee5406e
Merge "Remove ryu plugin"