socket API: Use pool index instead of handle in sock_delete handler

The socket API used the socket index handle directly in the sock_delete handler,
resulting in "unknown client id" warnings, and a failure in return for socket clients.

Change-Id: Ia69f740ce0f834cd9b62b7157243a1f42bcad765
Signed-off-by: Ole Troan <ot@cisco.com>