Remove hacks for supporting old versions of emerge from build_packages.
Remove hacks for supporting old versions of emerge from build_packages.

Now that emerge supports the flags needed to build packages, we don't
need to special-case it anymore.

BUG=chromium-os:24497
TEST=Trybot run.

Change-Id: I4ca1cd22c309f97fc14f1d7b9eede9128b7f0be1
Reviewed-on: https://gerrit.chromium.org/gerrit/13389
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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