Remove x86_64_v3 specific ref for RichFutureKernel histograms test

Christopher Rob Jonesrequested to merge
remove-histograms.ref.x86_64_v3 into master

Now we have migrated to Gaudi v39r2 which fixes some issue with histogram stats, there is no need for this platform specific ref any more

 > diff histograms.ref histograms.ref.x86_64_v3
 >

FYI @sponce

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