ParticleFlowFilter is never used

The following discussion from !3834 (merged) should be addressed:

• @sstahl started a discussion: (+2 comments)

  Does the old code cause segfaults in normal running?

ParticleFlowFilter is never used -- so should probably be removed. Unless it was never used because it would just crash due to double deletes... (which got fixed in !3834 (merged))