x/net/ipv6: remove unnecessary stubs

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/183750044

ipv6/sockopt_stub.go

@@ -8,34 +8,6 @@ package ipv6
 
 import "net"
 
-func getInt(fd int, opt *sockOpt) (int, error) {
-	return 0, errOpNoSupport
-}
-
-func setInt(fd int, opt *sockOpt, v int) error {
-	return errOpNoSupport
-}
-
-func getInterface(fd int, opt *sockOpt) (*net.Interface, error) {
-	return nil, errOpNoSupport
-}
-
-func setInterface(fd int, opt *sockOpt, ifi *net.Interface) error {
-	return errOpNoSupport
-}
-
-func getICMPFilter(fd int, opt *sockOpt) (*ICMPFilter, error) {
-	return nil, errOpNoSupport
-}
-
-func setICMPFilter(fd int, opt *sockOpt, f *ICMPFilter) error {
-	return errOpNoSupport
-}
-
 func getMTUInfo(fd int, opt *sockOpt) (*net.Interface, int, error) {
 	return nil, 0, errOpNoSupport
 }
-
-func setGroup(fd int, opt *sockOpt, ifi *net.Interface, grp net.IP) error {
-	return errOpNoSupport
-}