cros_generate_update_payload: kill old style update image generation
cros_generate_update_payload: kill old style update image generation

This disables an unwanted dependency.

Change-Id: I02752fa38cf5f64af366f1831a351b1b903563d0

BUG=n0ne
TEST=

Review URL: http://codereview.chromium.org/6349060

nicescale/scripts,gtank/scripts,bcwaldon/coreos-scripts,sigma/coreos-scripts,vmware/coreos-scripts,andrejro/coreos--scripts,crawford/scripts,gtank/scripts,BugRoger/scripts,nicescale/scripts,BugRoger/scripts,mjg59/scripts,glevand/coreos--scripts,BugRoger/scripts,mjg59/scripts,trnubo/scripts,bcwaldon/coreos-scripts,mischief/scripts,zhang0137/scripts,andrejro/coreos--scripts,marineam/coreos-scripts,coreos/scripts,mischief/scripts,trnubo/scripts,mischief/scripts,coreos/scripts,fivethreeo/scripts,gtank/scripts,marineam/coreos-scripts,crawford/scripts,smilart/scripts,sigma/coreos-scripts,crawford/scripts,vmware/coreos-scripts,nicescale/scripts,sigma/coreos-scripts,endocode/scripts,endocode/scripts,endocode/scripts,smilart/scripts,fivethreeo/scripts,zhang0137/scripts,trnubo/scripts,glevand/coreos--scripts,marineam/coreos-scripts,smilart/scripts,mjg59/scripts,zhang0137/scripts,fivethreeo/scripts,bcwaldon/coreos-scripts,vmware/coreos-scripts,andrejro/coreos--scripts