go.tools/cmd/godoc: remove useless code from index.go
go.tools/cmd/godoc: remove useless code from index.go

This code was moved to a throttle method on Corpus but I guess it was
never deleted.

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews
https://codereview.appspot.com/102350043

karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools