fix(algebra/algebra/restrict_scalars): Remove a bad instance (#8732)

This instance forms a non-defeq diamond with the following one
```lean
instance submodule.restricted_module' [module R M] [is_scalar_tower R S M] (V : submodule S M) :
  module R V :=
by apply_instance
```

The `submodule.restricted_module_is_scalar_tower` instance is harmless, but it can't exist without the first one so we remove it too.

Based on the CI result, this instance wasn't used anyway.

src/algebra/algebra/restrict_scalars.lean

@@ -107,14 +107,6 @@ rfl
 instance : is_scalar_tower R S (restrict_scalars R S M) :=
 ⟨λ r S M, by { rw [algebra.smul_def, mul_smul], refl }⟩
 
-instance submodule.restricted_module (V : submodule S M) :
-  module R V :=
-restrict_scalars.module R S V
-
-instance submodule.restricted_module_is_scalar_tower (V : submodule S M) :
-  is_scalar_tower R S V :=
-restrict_scalars.is_scalar_tower R S V
-
 end module
 
 section algebra