net: ipv4: Do not change the protocol type when sending

The ARP code has set the protocol type of the packet to
0x806, so do not change it when preparing to send to 0x800
which is the IP protocol type. Lets trust the previously
called functions to set the ptype correctly and do not set
it here.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>