net/dhcpv4: Call net_if_ipv4_addr_rm

The DHCPv4 client obtains resources from the server on a lease.  Once
a lease expires those resources must be relinquished.

Update the DHCPv4 client to explicitly remove leased resources from
the network stack once a lease has expired.

Change-Id: I5d3b7a02e463041cfdee1d104f5962498bdd6a30
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>