Remove dead code, pep8 cleanups for dask.bag
Remove dead code, pep8 cleanups for dask.bag

- Remove some dead imports and variables
- A few pep8 cleanups

dask/dask,mikegraham/dask,jcrist/dask,cowlicks/dask,blaze/dask,jakirkham/dask,mraspaud/dask,dask/dask,jcrist/dask,mrocklin/dask,gameduell/dask,cpcloud/dask,mrocklin/dask,ContinuumIO/dask,blaze/dask,chrisbarber/dask,ContinuumIO/dask,mraspaud/dask,jakirkham/dask