Run dead code elimination by default in GenBCode

This was disabled by mistake. Settings are still a challenge.
This fixes the bcode-delambdafy-method build
(https://scala-webapps.epfl.ch/jenkins/view/2.11.x/job/scala-nightly-genbcode-2.11.x/)
Merge pull request #4211 from lrytz/bcode-build

Run dead code elimination by default in GenBCode