[range.join.with.iterator] Simplify `operator--` with *`as-lvalue`*

[JohelEGP]

This drafting note accompanies https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2770r0.html#pnum_26:

[ Drafting note: The specification of operator-- in paragraph 14 can be editorially simplified using the new as-lvalue helper; that cleanup is not included in this paper as there is no normative impact. ]

See operator--: https://eel.is/c++draft/range.join.with.iterator#itemdecl:10.

The PR which adds as-lvalue is #6128.