openflow: Delete icmp_type and icmp_code macros.

openflow: Delete icmp_type and icmp_code macros.

These macros caused trouble if datapath-protocol.h was included before
openflow.h.  Later references to the icmp_type and icmp_code members of
struct ovs_key_icmp caused compiler errors, because the macros caused them
to try to refer to nonexistent tp_src and tp_dst members in those
structures.