syscall: remove sysctl workaround for openbsd
syscall: remove sysctl workaround for openbsd

Now that Go will no longer work on OpenBSD versions prior to 5.1,
remove the sysctl workaround that was needed for 5.0 and earlier.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6116050

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang