Avoid single-interaction gRoo refetch

[jdkio]

This case is already handled before. So we can remove this code and get rid of the error mentioned in issue #251.

Truth_Spill->Show(0)
======> EVENT:0
 EventNo         = 0
 SpillNo         = 0
 RunNo           = 1000000101
 IsCC            = 1
 Interaction     = nu:14;tgt:1000260560;N:2112;proc:Weak[CC],QES;
 TruthInfoIndex  = 0
 TruthInfoNSlices = 2
 nPrimaryVertices = 1
 HasPileup       = 0
 NeutrinoPDG     = 14
 NeutrinoP4      = 0.0227525, 
                  -0.297354, 2.8991, 2.9144
 NeutrinoX4      = 2758.56, 
                  -747.633, 12699.9, 9.81494e-07