net: ip: dhcpv4: remove address on interface down

Any received address is no longer valid once the interface goes down.
Leaving the address assigned results in the L4 interface transitioning
through the following on reconnection:
 UP: Interface is connected
 DOWN: Old address is removed by DHCP
 UP: New address is re-added by DHCP

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>