revert commenting out java version error

Use LEGACY_USE_JAVA6=true for 1.6. There's a check for this
a few lines above this version error. It's good to have this here
so one will be notified if he/she has undesired JDK version.

Change-Id: I0e5a6e212c4125e57ef2c03a7e7acec8f72f1744