json: remove old optimization that inlining covers now
json: remove old optimization that inlining covers now

Benchmarks look the same.

R=golang-dev, dsymonds, rsc
CC=golang-dev
http://codereview.appspot.com/5572080
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go