unix: remove unused readlen and writelen

These were originally added when copying over files from package
syscall. However, their use was removed in CL 129500043. New ports kept
adding these wrappers without using them. They are still unused, so
remove them.

Change-Id: I2ffffe98a5c999d23b63d1f04ee4ad8f807ee736
Reviewed-on: https://go-review.googlesource.com/c/sys/+/531356
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/mksyscall.go

@@ -373,7 +373,7 @@ func main() {
 			text += "}\n\n"
 
 			if *libc && !trampolines[libcFn] {
-				// some system calls share a trampoline, like read and readlen.
+				// Some system calls share a trampoline.
 				trampolines[libcFn] = true
 				// Declare assembly trampoline address.
 				text += fmt.Sprintf("var libc_%s_trampoline_addr uintptr\n\n", libcFn)