Zap outdated nd6_free() comment about static

Zap outdated nd6_free() comment about static

Added in 2002 r1.48 "sync with latest KAME [...]" along the attribute,
but nd6_free() became a global void function in 2017 r1.212.

Afaik static kernel functions are avoided to aid ddb'ugging and I
presume the "significant changes in the kernel" bits of the comment stem
from something 20 years ago no longer holding true today.

Afterall, this change has been safe for five years.

OK claudio