<concepts>: LWG-3175 has been accepted, so we should remove commented-out code

[StephanTLavavej]

Our implementation of swappable_with speculatively implemented the proposed resolution for LWG-3175 (a Library Issue, which is a bug report against the C++ Standard):

STL/stl/inc/concepts

Lines 183 to 195 in e034296

	template <class _Ty1, class _Ty2>
	concept swappable_with =
	#if 1 // Implement the PR of LWG-3175
	common_reference_with<_Ty1, _Ty2>
	#else // ^^^ LWG-3175 / N4810 vvv
	common_reference_with<const remove_reference_t<_Ty1>&, const remove_reference_t<_Ty2>&>
	#endif // select LWG-3175 vs. N4810
	&& requires(_Ty1&& __t, _Ty2&& __u) {
	_RANGES swap(static_cast<_Ty1&&>(__t), static_cast<_Ty1&&>(__t));
	_RANGES swap(static_cast<_Ty2&&>(__u), static_cast<_Ty2&&>(__u));
	_RANGES swap(static_cast<_Ty1&&>(__t), static_cast<_Ty2&&>(__u));
	_RANGES swap(static_cast<_Ty2&&>(__u), static_cast<_Ty1&&>(__t));
	};

At the February 2020 meeting in Prague, LWG-3175 was resolved for C++20. Therefore, we should keep line 186 here, but remove lines 185 and 187-189.