Avoid std::terminate for R package. (#7661)

This is part of CRAN policies.

src/tree/hist/evaluate_splits.h

@@ -114,9 +114,6 @@ template <typename GradientSumT, typename ExpandEntry> class HistEvaluator {
           left_sum.SetSubstract(parent.stats, right_sum);
           break;
         }
-        default: {
-          std::terminate();
-        }
       }
     };