Remove redundant Return node after processing an ASR through subroutine_from_function pass

[anutosh491]

From #2419

When we go from

def func() -> S:
    return pi

To something like

def func(r: Out[S]) -> None:
    r = pi

Or any cases like this where the subroutine_from_function pass is involved, we don't get rid of the redundant Return node in the function body.

                                    (=
                                        (Var 3 _lpython_return_variable)
                                        (IntrinsicScalarFunction
                                            SymbolicPi
                                            []
                                            0
                                            (SymbolicExpression)
                                            ()
                                        )
                                        ()
                                    )
                                    (Return)

Whereas x->m_return_var points to nullptr so we don't need this node.

[certik]

Looks like that this is fixed now.