Update MergeMcEventCollTool.cxx: remove the obsolete conditions

Andrii Verbytskyirequested to merge
averbyts-master-patch-79002 into master

Update MergeMcEventCollTool.cxx: remove the obsolete conditions

@jchapman, my suspects are that the code can be updated like that as the way to find out the interaction vertex should be just using the beam particles.

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