remove ControlSystem

It was only half completed, and now it's been folded into ODESystem itself so it has no place anymore. Bye bye.

The only thing we should try to keep is the `runge_kutta_discretize`, which instead should be ODESystem -> OptimizationSystem transformation

Author: ChrisRackauckas

docs/pages.jl

@@ -8,7 +8,6 @@ pages = [
                                          "tutorials/nonlinear.md",
                                          "tutorials/optimization.md",
                                          "tutorials/stochastic_diffeq.md",
-                                         "tutorials/nonlinear_optimal_control.md",
                                          "tutorials/parameter_identifiability.md"],
     "ModelingToolkitize Tutorials" => Any["mtkitize_tutorials/modelingtoolkitize.md",
                                           "mtkitize_tutorials/modelingtoolkitize_index_reduction.md",
@@ -25,7 +24,6 @@ pages = [
                           "systems/JumpSystem.md",
                           "systems/NonlinearSystem.md",
                           "systems/OptimizationSystem.md",
-                          "systems/ControlSystem.md",
                           "systems/PDESystem.md"],
     "comparison.md",
     "internals.md",

docs/src/systems/ControlSystem.md

@@ -136,8 +136,6 @@ include("systems/nonlinear/modelingtoolkitize.jl")
 
 include("systems/optimization/optimizationsystem.jl")
 
-include("systems/control/controlsystem.jl")
-
 include("systems/pde/pdesystem.jl")
 
 include("systems/sparsematrixclil.jl")
@@ -172,13 +170,11 @@ export AutoModelingToolkit
 export SteadyStateProblem, SteadyStateProblemExpr
 export JumpProblem, DiscreteProblem
 export NonlinearSystem, OptimizationSystem
-export ControlSystem
 export alias_elimination, flatten
 export connect, @connector, Connection, Flow, Stream, instream
 export isinput, isoutput, getbounds, hasbounds, isdisturbance, istunable, getdist, hasdist,
        tunable_parameters, isirreducible
 export ode_order_lowering, dae_order_lowering, liouville_transform
-export runge_kutta_discretize
 export PDESystem
 export Differential, expand_derivatives, @derivatives
 export Equation, ConstrainedEquation