The causesIPv6Crash helper function was only needed on macOS 10.8 and
before. However, Go 1.11 requires at least macOS 10.11, so drop the
check altogether. Also remove the unused implementations for all other
geese.

Change-Id: I7459f942374e7e2e6c14b787e2dfc493de67bedc
Reviewed-on: https://go-review.googlesource.com/c/156497
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>