cmd/pack: remove unused paging logic
cmd/pack: remove unused paging logic

This is the remainder of http://codereview.appspot.com/4601051.

Partially addresses issue 2705.

R=golang-dev, r, bradfitz, minux.ma
CC=golang-dev
http://codereview.appspot.com/6354066

firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang