refactor: remove implicit_discrete_system.jl

Author: AayushSabharwal

src/ModelingToolkit.jl

@@ -175,8 +175,6 @@ include("systems/diffeqs/first_order_transform.jl")
 include("systems/diffeqs/modelingtoolkitize.jl")
 include("systems/diffeqs/basic_transformations.jl")
 
-include("systems/discrete_system/implicit_discrete_system.jl")
-
 include("systems/jumps/jumpsystem.jl")
 
 include("systems/pde/pdesystem.jl")
@@ -264,7 +262,7 @@ export DAEFunctionExpr, DAEProblemExpr
 export SDEFunction, SDEFunctionExpr, SDEProblemExpr
 export SystemStructure
 export DiscreteProblem, DiscreteFunction, DiscreteFunctionExpr
-export ImplicitDiscreteSystem, ImplicitDiscreteProblem, ImplicitDiscreteFunction,
+export ImplicitDiscreteProblem, ImplicitDiscreteFunction,
        ImplicitDiscreteFunctionExpr
 export JumpSystem
 export ODEProblem, SDEProblem