net/smtp: allow PLAIN auth when advertised

The smtp package originally allowed PLAIN whenever, but then
the TLS check was added for paranoia, but it's too paranoid:
it prevents using PLAIN auth even from localhost to localhost
when the server advertises PLAIN support.

This CL also permits the client to send PLAIN if the server
advertises it.

Fixes Issue 5184

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8279043