net: acd: Avoid removing auto IPv4 address twice

When the network interface goes down, we call
net_ipv4_autoconf_reset() which removes the autoaddress
from the network interface.
The net_ipv4_autoconf_reset() is also called when ACD is started
in which case we could see this error message

<dbg> net_if_start_acd: Starting ACD for iface 2
<err> net_if: iface 2 addr 169.254.174.230 (net_if_ipv4_addr_rm():4625)
<dbg> net_if_ipv4_addr_rm: Address 169.254.174.230 not found (-22)

This error is superfluous and not needed. So before trying to
remove the address, check if the interface already has it set and
only then remove it.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
(cherry picked from commit bd4c0b74)