Remove mlnx plugin

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

Mellanox recommends users to use mlnx or SR-IOV MD

DocImpact
Closes-Bug: #1386539

Change-Id: I7d109bf62f71b2996aef4e0002f447ca3439a5cf
Signed-off-by: Moshe Levi <moshele@mellanox.com>
Merge "Remove mlnx plugin"