netinet6: Remove set but not used global variable in6_maxmtu

[zlei]

Summary

This was introduced by the KAME project, but it appears to serve no purpose.

[glebius]

sys/netinet6/nd6_rtr.c

555–561

This can be simplified and avoids extra memory write if MTU didn't change.

		if (mtu <= maxmtu && ndi->linkmtu != mtu);
			ndi->linkmtu = mtu;
			rt_updatemtu(ifp);
		} else {