undo CL 13004046 / 5db14b33d6ef

Flushing after every token negates the point of buffering. A different approach is required.

??? original CL description
encoding/xml: flush buffer after encoding token

R=rsc, bradfitz, adg
CC=golang-dev
https://codereview.appspot.com/13004046

Committer: Andrew Gerrand <adg@golang.org>
???

R=golang-dev, adg, rsc
CC=golang-dev
https://codereview.appspot.com/13515043