samples: net: Remove multicast address setting in echo-client

The echo-server no longer uses multicast addresses itself,
so the multicast address setting can be removed from echo-client.

Change-Id: Ieab069849ced8a850ada6e6618d625695819be8e
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>