wifi: network_remove: Clear network pointer in wifi

wifi: network_remove: Clear network pointer in wifi

When a network is destroyed we also have to remove the pointer
from the wifi data structure.

connmand[25303]: plugins/wifi.c:network_remove() network 0x69c350
connmand[25303]: src/network.c:connman_network_unref() network 0x69c350 name bmwnet02 refcount 1
connmand[25303]: src/network.c:connman_network_unref() network 0x69c350 name bmwnet02 refcount 0
connmand[25303]: src/network.c:network_destruct() network 0x69c350 name XXXXX

[...]

connmand[25303]: plugins/wifi.c:interface_state() wifi 0x699d60 interface state 1
connmand[25303]: plugins/wifi.c:is_idle() state 9
connmand[25303]: src/network.c:connman_network_set_associating() network 0x69c350 associating 0
connmand[25303]: src/network.c:connman_network_set_connected() network 0x69c350 connected 0

Fixes BMC#22307