`<xstring>`: Remove spurious `operator>>(basic_istream&&, basic_string&)`

[StephanTLavavej]

Noticed by S. B. Tam (@cpplearner) in #2025 (comment).

⚠️ We should wait until #2025 is merged before dealing with this.

The issue is that <xstring> defines an operator>>(basic_istream&&, basic_string&) which doesn't appear in the Standard:

STL/stl/inc/xstring

Lines 4989 to 4991 in e745bad

	template <class _Elem, class _Traits, class _Alloc>
	basic_istream<_Elem, _Traits>& operator>>(
	basic_istream<_Elem, _Traits>&& _Istr, basic_string<_Elem, _Traits, _Alloc>& _Str) {

We should (1) remove this overload and (2) move its guts to operator>>(basic_istream&, basic_string&) which appears in the Standard at WG21-N4892 21.3.4.4 [string.io]/1:

STL/stl/inc/xstring

Lines 5038 to 5042 in e745bad

	template <class _Elem, class _Traits, class _Alloc>
	basic_istream<_Elem, _Traits>& operator>>(
	basic_istream<_Elem, _Traits>& _Istr, basic_string<_Elem, _Traits, _Alloc>& _Str) {
	return _STD move(_Istr) >> _Str;
	}

Rvalue stream extraction should continue to work (i.e. with no test changes necessary) due to our implementation of 29.7.4.6 [istream.rvalue] operator>>(Istream&&, T&&):

STL/stl/inc/istream

Lines 886 to 891 in e745bad

	template <class _Istr, class _Ty,
	enable_if_t<conjunction_v<is_convertible<_Istr*, ios_base*>, _Can_stream_in<_Istr, _Ty>>, int> = 0>
	_Istr&& operator>>(_Istr&& _Is, _Ty&& _Val) { // extract from rvalue stream
	_Is >> _STD forward<_Ty>(_Val);
	return _STD move(_Is);
	}