MAINTAINERS: ipv6: retire Hideaki Yoshifuji

We very rarely hear from Hideaki Yoshifuji and the IPv4/IPv6
entry covers a lot of code. Asking people to CC someone who
rarely responds feels wrong.

Note that Hideaki Yoshifuji already has an entry in CREDITS
for IPv6 so not adding another one.

Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>