Remove undefined exports

Author: ChrisRackauckas

src/ModelingToolkit.jl

@@ -331,7 +331,7 @@ export debug_system
 #export ContinuousClock, Discrete, sampletime, input_timedomain, output_timedomain
 #export has_discrete_domain, has_continuous_domain
 #export is_discrete_domain, is_continuous_domain, is_hybrid_domain
-export Sample, Hold, Shift, ShiftIndex, sampletime, SampleTime, Next, Prev
+export Sample, Hold, Shift, ShiftIndex, sampletime, SampleTime
 export Clock, SolverStepClock, TimeDomain
 
 export MTKParameters, reorder_dimension_by_tunables!, reorder_dimension_by_tunables