netinet6: Remove a set but not used global variable in6_maxmtu

and its setter in6_setmaxmtu().

This variable was introduced by the KAME projec [1]. It holds the max
IPv6 MTU through all the interfaces, but is never used anywhere.

[1] 82cd038d51e2 KAME netinet6 basic part(no IPsec,no V6 Multicast

Forwarding, no UDP/TCP for IPv6 yet)

Reviewed by: glebius
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D49357