x/net/ipv4: remove unnecessary stubs

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/181200043

ipv4/sockopt_stub.go

@@ -6,24 +6,6 @@
 
 package ipv4
 
-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 setGroup(fd int, opt *sockOpt, ifi *net.Interface, ip net.IP) error {
-	return errOpNoSupport
-}