CL 92475 added WSAEMSGSIZE const to syscall package. But there
is already copy of WSAEMSGSIZE in internal/syscall/windows.
So delete syscall.WSAEMSGSIZE

Change-Id: I0b81fa5dcf846887a0cb27d8bbd7e250860627b5
Reviewed-on: https://go-review.googlesource.com/106956
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>