TrackToMCParticleRelations - Remove empty relations warning

Removes warning counter for empty relations. In practise not much use.

Should address 'Unexpected WARNING' failures such as that at

https://lhcb-nightlies.web.cern.ch/logs/tests/nightly/lhcb-gaudi-head/2506/x86_64-centos7-gcc9-opt/Moore/

Will require ref updates in Rec and Moore.