FreeBSD/src b141dd5 (r354751)share/doc/IPv6 IMPLEMENTATION, sys/netinet ip6.h

Remove now unused IPv6 macros and update docs.

After r354748-354750 all uses of the IP6_EXTHDR_CHECK() and
IP6_EXTHDR_GET() macros are gone from the kernel.  IP6_EXTHDR_GET0()
was unused.  Remove the macros and update the documentation.

Sponsored by:   Netflix
DeltaFile
+0-84sys/netinet/ip6.h
+0-14share/doc/IPv6/IMPLEMENTATION
+0-982 files

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