Chore(LinearAlgebra/Basis/VectorSpace): Remove the single instance of `Module.Submodule` namespace in mathlib

[Shamrock-Frost]

Remove only usage of Module.Submodule.

---

The existence of this namespace makes it awkward to do open Submodule (...) or open Submodule renaming ... while within the Module namespace, and since it's only used in one place I assume it's used in error here.