remove useless looping in pipeline

previously, the pipeline filtered at the publisher stage. this
logic has been moved to an earlier stage yet we still do processing
at publisher.

this patch removes legacy code. at worse, it will have no effect on
performance. at best, we publish quicker as we don't loop and
groupby needlessly for each batch of samples.

Change-Id: I2171c067bd8bb293a8478756f8b4938f312614a1
Closes-Bug: #1401176
Merge "remove useless looping in pipeline"