net: remove another unguarded sysfile == nil check

Putting aside the unguarded access to fd.sysfile, the condition will never be true as fd.incref above handles the closed condition.

R=mikioh.mikioh, dvyukov
CC=golang-dev
https://golang.org/cl/6845062

src/pkg/net/fd_unix.go

@@ -509,10 +509,6 @@ func (fd *netFD) Write(p []byte) (int, error) {
 		return 0, err
 	}
 	defer fd.decref()
-	if fd.sysfile == nil {
-		return 0, syscall.EINVAL
-	}
-
 	var err error
 	nn := 0
 	for {