net: samples: Fix stale yaip references

Fixes two references to yaip that were were left behind after net/yaip
was renamed to subsys/net/ip. The zperf sample needs net_private.h, so
the include path is updated. The dhcpv4_client sample doesn't need it,
so the path is removed.

Change-Id: I189f1b58191524330254a5b7b31a9a4974956728
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>