Remove unnecessary #if guards
Remove unnecessary #if guards

ipv4/uip.c will not even be include in the list of sources in the first place if we are building with NETSTACK_CONF_WITH_IPV6 (CONTIKI_WITH_IPV6). Thus, there is no longer a need to wrap the entire source code in an #if guard.

Closes #935

bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki