Remove unused ES6TranspilerPreprocessor
Remove unused ES6TranspilerPreprocessor

The more appropriate place for this functionality seems to be the
ES6ConcatenatorCompiler.

gabesoft/broccoli,osxi/broccoli,broccolijs/broccoli,broccolijs/broccoli,ixisystems/broccoli,rwjblue/broccoli,gabesoft/broccoli,osxi/broccoli,gabesoft/broccoli,ember-cli/broccoli,ember-cli/broccoli,ember-cli/broccoli,broccolijs/broccoli,jnicklas/broccoli,ixisystems/broccoli,broccolijs/broccoli,osxi/broccoli