chore(Probability): remove TODO on characteristic functions (leanprover-community#40595)

mathlib4 now has Fourier transforms and the characteristic function is defined at [`MeasureTheory.charFun`](https://leanprover-community.github.io/mathlib4_docs/Mathlib/MeasureTheory/Measure/CharacteristicFunction/Basic.html#MeasureTheory.charFun).

Author: LLaurance

Mathlib/Probability/Density.lean

@@ -43,12 +43,6 @@ random variables with this distribution.
 * `MeasureTheory.pdf.IsUniform.integral_eq` : If `X` follows the uniform distribution with
   its pdf having support `s`, then `X` has expectation `(λ s)⁻¹ * ∫ x in s, x dx` where `λ`
   is the Lebesgue measure.
-
-## TODO
-
-Ultimately, we would also like to define characteristic functions to describe distributions as
-it exists for all random variables. However, to define this, we will need Fourier transforms
-which we currently do not have.
 -/
 
 @[expose] public section