encoding/xml: remove Marshaler support

encoding/xml: remove Marshaler support

Marshaler has a number of open areas that need
further thought (e.g. it doesn't handle attributes,
it's supposed to handle tag names internally but has
no information to do so, etc).

We're removing it now and will bring it back with an
interface that covers these aspects, after Go 1.

Related to issue 2771, but doesn't fix it.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5574057