exp/norm: LastBoundary is used in preparation for an append operation. It seems

therefore unlikely that there is a good use for its string version
LastBoundaryInString. Yet, the implemenation of this method would complicate
things a bit as it would require the introduction for another interface and
some duplication of code. Removing it seems a better choice.

R=r
CC=golang-dev
https://golang.org/cl/5182044

src/pkg/exp/norm/normalize.go

@@ -379,12 +379,6 @@ func lastBoundary(fd *formInfo, b []byte) int {
 	return i
 }
 
-// LastBoundaryInString returns the position i of the last boundary in s
-// or -1 if s contains no boundary.
-func (f Form) LastBoundaryInString(s string) int {
-	panic("not implemented")
-}
-
 // decomposeSegment scans the first segment in src into rb.
 // It returns the number of bytes consumed from src.
 // TODO(mpvl): consider inserting U+034f (Combining Grapheme Joiner)