unix: remove Mclpool from openbsd types

This struct was removed from the OpenBSD headers more than eight years
ago (r1.157 of net/if.h).

Change-Id: Ic2d1b3325dc5d00859584b3756a9f7c56b4ec417
Reviewed-on: https://go-review.googlesource.com/c/sys/+/459498
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

unix/types_openbsd.go

@@ -209,8 +209,6 @@ type RtMsghdr C.struct_rt_msghdr
 
 type RtMetrics C.struct_rt_metrics
 
-type Mclpool C.struct_mclpool
-
 // Berkeley packet filter
 
 const (