runtime: remove obsolete SELinux execmem comment

We don't have executable memory anymore.

Change-Id: I9835f03a7bcd97d809841ecbed8718b3048bfb32
Reviewed-on: https://go-review.googlesource.com/4681Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/runtime/mem_linux.go

@@ -54,7 +54,6 @@ func sysAlloc(n uintptr, stat *uint64) unsafe.Pointer {
 	if uintptr(p) < 4096 {
 		if uintptr(p) == _EACCES {
 			print("runtime: mmap: access denied\n")
-			print("if you're running SELinux, enable execmem for this process.\n")
 			exit(2)
 		}
 		if uintptr(p) == _EAGAIN {