The net package consists of thin three layers like the follwoing;

- Exposed API, that contains net.Dial, net.DialUDP, net.DialUnix
- Socket and network file descriptor, that contains net.netFD and
  its methods, helper functions such as dialUDP, dialUnix
- Network pollster, that contains net.pollDesc and its methods

This CL removes redundant argument check which is already done by
API layer.

R=golang-dev, dave, bradfitz
CC=golang-dev
https://golang.org/cl/13092043