fixup! refactor: use clock from SciMLBase, fix tests

Author: AayushSabharwal

src/clock.jl

@@ -115,7 +115,4 @@ function Base.showerror(io::IO, cie::ClockInferenceException)
     print(io, "ClockInferenceException: ", cie.msg)
 end
 
-struct IntegerSequence
-    t::Union{Nothing, Symbolic}
-    IntegerSequence(t::Union{Num, Symbolic}) = new(value(t))
-end
+struct IntegerSequence end

test/clock.jl

@@ -72,7 +72,7 @@ sss, = ModelingToolkit._structural_simplify!(deepcopy(tss[1]), (inputs[1], ()))
 @test isempty(equations(sss))
 d = Clock(dt)
 k = ShiftIndex(d)
-@test observed(sss) == [yd(k + 1) ~ Sample(t, dt)(y); r(k + 1) ~ 1.0;
+@test observed(sss) == [yd(k + 1) ~ Sample(dt)(y); r(k + 1) ~ 1.0;
        ud(k + 1) ~ kp * (r(k + 1) - yd(k + 1))]
 
 d = Clock(dt)