The argument was never used for anything else

Previously, newlines after the last list/attrs/let-in item would not be
preserved. So e.g.

    [
      "foo"

    ]

would wrongly turn into

    [
      "foo"
    ]

Subsequently it would turn into

    [ "foo" ]

This commit fixes it, such that the original is preserved in this case.

Whereas previously all the newlines were stripped already during
parsing, in this new commit all newlines are preserved, which both fixes
the above problem and simplifies the code.

This does introduce one odd edge case due to interaction with another
feature. That feature is that comments before a let-in's `in` part, get
moved below it. The following:

    let
      x = 10;
    # X
    in
    x

Gets reformatted into

    let
      x = 10;
    in
    # X
    x

Due to how the new code preserves newlines, those newly also get moved
further down. So whereas previously the following:

    let
      x = 10;

    in
    x

Would wrongly remove the newline, turning it into:

    let
      x = 10;
    in
    x

With this change it won't anymore, but it will move the newline down:

    let
      x = 10;
    in

    x

While this could be handled with a special case, I'm not sure if that's
worth it. It might be better to rethink this moving of comments further
down idea, I'm not sure if that's necessary anymore.

This fixes the odd case introduced in the parent commit