refactor: remove AbstractODESystem

Author: AayushSabharwal

src/ModelingToolkit.jl

@@ -127,7 +127,6 @@ TODO
 abstract type AbstractSystem end
 abstract type AbstractTimeDependentSystem <: AbstractSystem end
 abstract type AbstractTimeIndependentSystem <: AbstractSystem end
-abstract type AbstractODESystem <: AbstractTimeDependentSystem end
 abstract type AbstractMultivariateSystem <: AbstractSystem end
 abstract type AbstractOptimizationSystem <: AbstractTimeIndependentSystem end
 abstract type AbstractDiscreteSystem <: AbstractTimeDependentSystem end