runtime: remove reference to openbsd kern.rthreads sysctl

The kern.rthreads sysctl has not existed for a long time - there is no way to
disable rthreads and __tfork no longer returns ENOTSUP.

Change-Id: Ia50ff01ac86ea83358e72b8f45f7818aaec1e4b1
Reviewed-on: https://go-review.googlesource.com/7490Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/os1_openbsd.go

@@ -117,9 +117,6 @@ func newosproc(mp *m, stk unsafe.Pointer) {
 
 	if ret < 0 {
 		print("runtime: failed to create new OS thread (have ", mcount()-1, " already; errno=", -ret, ")\n")
-		if ret == -_ENOTSUP {
-			print("runtime: is kern.rthreads disabled?\n")
-		}
 		throw("runtime.newosproc")
 	}
 }