Remove FD(), type asserting the ReadWriteCloser is sufficient

Author: Arceliar

if.go

@@ -3,7 +3,6 @@ package water
 import (
 	"errors"
 	"io"
-	"os"
 )
 
 // Interface is a TUN/TAP interface.
@@ -78,8 +77,3 @@ func (ifce *Interface) IsTAP() bool {
 func (ifce *Interface) Name() string {
 	return ifce.name
 }
-
-// FD returns the underlying File object, which will be needed to ioctl later
-func (ifce *Interface) FD() *os.File {
-	return ifce.ReadWriteCloser.(*os.File)
-}