Revert "sort: improve average quicksort performance"

Broke the build: http://build.golang.org/log/8159de7e0d6f3832da394c310975ddd4c4c74627
(cmd/gofmt TestRewrite)

This reverts commit 6f6b2f04.

Change-Id: Ifd46b0b76c30b0a568521eaaf5ef8968a9549bf5
Reviewed-on: https://go-review.googlesource.com/17383


Reviewed-by: Russ Cox <rsc@golang.org>