Delete support for build_image --preserve
Delete support for build_image --preserve

A survey of users on chromium-os-dev@chromium.org found no-one uses
the option.  Furthermore, the option was meant merely for
performance improvements, but parallel emerge is more effective for
that purpose.

BUG=None
TEST=build and boot image

Change-Id: I42ab4a9cb5d70f1181915e957a5cc9fc4e3dbed7
Reviewed-on: http://gerrit.chromium.org/gerrit/5356
Reviewed-by: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>
Tested-by: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>

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