go.net/ipv6: simplfy build tags

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/91020043

ipv6/syscall_nosplit4_linux_386.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build go1.2 go1.3
+// +build go1.2
 
 package ipv6
 

ipv6/syscall_nosplit7_linux_386.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build go1.1,!go1.2,!go1.3
+// +build go1.1,!go1.2
 
 package ipv6