structural_simplify doesn't work with discrete systems yet

Author: YingboMa

src/systems/abstractsystem.jl

@@ -1032,17 +1032,13 @@ simplification will allow models where `n_states = n_equations - n_inputs`.
 """
 function structural_simplify(sys::AbstractSystem, io = nothing; simplify = false, kwargs...)
     sys = expand_connections(sys)
+    sys isa DiscreteSystem && return sys
     state = TearingState(sys)
     has_io = io !== nothing
     has_io && markio!(state, io...)
     state, input_idxs = inputs_to_parameters!(state, io)
     sys, ag = alias_elimination!(state; kwargs...)
-    #ag = AliasGraph(length(ag))
-    # TODO: avoid construct `TearingState` again.
-    #state = TearingState(sys)
-    #has_io && markio!(state, io..., check = false)
     check_consistency(state, ag)
-    #find_solvables!(state; kwargs...)
     sys = dummy_derivative(sys, state, ag; simplify)
     fullstates = [map(eq -> eq.lhs, observed(sys)); states(sys)]
     @set! sys.observed = topsort_equations(observed(sys), fullstates)