ping: Remove ifndef icmp_data guards

Early versions of <netinet/ip_icmp.h> [1] did not have icmp_data
defined, but FreeBSD has always had. Remove these guards.

NetBSD [2] and OpenBSD [3] have already removed them.

[1]: https://www.tuhs.org/cgi-bin/utree.pl?file=4.2BSD/usr/src/sys/netinet/ip_icmp.h
[2]: https://github.com/NetBSD/src/commit/203dfd34867991fd002f747d74a96f26ae80d41c
[3]: https://github.com/openbsd/src/commit/d83449c83c03b86e72d642dc92bcea434310435a

Reviewed by: markj
MFC after: 1 week