Previously we had a bitmap to check whether or not a byte
appears in a string should be replaced. But we don't actually
need a separate bitmap for that purpose. Removing the bitmap
makes the code simpler.

LGTM=dave, iant, nigeltao
R=golang-codereviews, dave, gobot, nigeltao, iant, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/110100043