Remove obsolete CHROMEOS_REVISION setting in enter_chroot.
Remove obsolete CHROMEOS_REVISION setting in enter_chroot.

BUG=chromium-os:11647
TEST=enter_chroot, build_image

Change-Id: Ib97e27fd0e6292ec7504dbc4312254bc574ba171

Review URL: http://codereview.chromium.org/6598056

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