unix: remove syscall constants on Solaris

SYS_EXECVE and SYS_FCNTL were needed for compatibility reasons in
syscall, but they are not needed in x/sys/unix.

Change-Id: Ic355cf8785c83be51bc91ba84401bc969f8ae6b5
Reviewed-on: https://go-review.googlesource.com/84995
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/zsysnum_solaris_amd64.go

-// Copyright 2014 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build amd64,solaris
-
-package unix
-
-// TODO(aram): remove these before Go 1.3.
-const (
-	SYS_EXECVE = 59
-	SYS_FCNTL  = 62
-)