lib: drop heuristic IPv6 address recognition

lib: drop heuristic IPv6 address recognition

* command.c: (cmd_ipv6_match) Drop IPv6 address recognition
  heuristics and solely rely on inet_pton, because strings
  like "abcd" were mistaken for IPv6 addresses.
  This affects e.g. the command "neighbour WORD peer-group",
  which won't work with words consisting of up to 4 characters
  between 'a' and 'f' and digits.

From: Roman Hoog Antink <rha@open.ch>
[full delete instead of #if 0]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>