syscall: make SocketDisableIPv6 obsolete

[mikioh]

It seems that having the state of IPv6 functionality in several places makes things
getting worse; e.g., issue #7687.

[mikioh]

Comment 1:

https://golang.org/cl/83060046/

[gopherbot]

Comment 2:

CL https://golang.org/cl/108930044 mentions this issue.

[rsc]

Comment 3:

I disagree. See my comments on issue #7687. Maybe we need a different way to make it
work, but I'm not even sure about that. I think issue #7687 is a real race.

Status changed to WorkingAsIntended.