Remove unnecessary error message in Arbor

[mark-grimes]

Currently the Arbor cluster maker prints an error every event stating how many clusters were found. This changes that to an info message.

@lgray

[mark-grimes]

Already tested privately with 15600, ready to merge but going to wait a while in case there are any complaints from the HGCal dev team (@lgray, @vandreev11, @pfs, @kpedro88).

[cmsbuild]

A new Pull Request was created by @mark-grimes (Mark Grimes) for CMSSW_6_2_X_SLHC.

Remove unnecessary error message in Arbor

It involves the following packages:

RecoParticleFlow/PFClusterProducer

@cmsbuild, @cvuosalo, @slava77 can you please review it and eventually sign? Thanks.
@mmarionncern, @bachtis, @lgray this is something you requested to watch as well.
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@fratnikov, @mark-grimes you are the release manager for this.
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[mark-grimes]

merge