refactor: remove Substitutions

Author: AayushSabharwal

src/structural_transformation/StructuralTransformations.jl

@@ -23,7 +23,6 @@ using ModelingToolkit: System, AbstractSystem, var_from_nested_derivative, Diffe
                        ExtraVariablesSystemException,
                        vars!,
                        IncrementalCycleTracker, add_edge_checked!, topological_sort,
-                       invalidate_cache!, Substitutions, get_or_construct_tearing_state,
                        filter_kwargs, lower_varname_with_unit,
                        lower_shift_varname_with_unit, setio, SparseMatrixCLIL,
                        get_fullvars, has_equations, observed,

src/structural_transformation/symbolics_tearing.jl

@@ -734,7 +734,6 @@ function update_simplified_system!(
 
     @set! sys.eqs = neweqs
     @set! sys.observed = obs
-    # @set! sys.substitutions = Substitutions(subeqs, deps)
 
     # Only makes sense for time-dependent
     if ModelingToolkit.has_schedule(sys)

src/systems/abstractsystem.jl

@@ -197,13 +197,6 @@ end
 
 const MTKPARAMETERS_ARG = Sym{Vector{Vector}}(:___mtkparameters___)
 
-mutable struct Substitutions
-    subs::Vector{Equation}
-    deps::Vector{Vector{Int}}
-    subed_eqs::Union{Nothing, Vector{Equation}}
-end
-Substitutions(subs, deps) = Substitutions(subs, deps, nothing)
-
 Base.nameof(sys::AbstractSystem) = getfield(sys, :name)
 description(sys::AbstractSystem) = has_description(sys) ? get_description(sys) : ""