Mostly revert conditional inclusion change (for now) due to a circular inclusion in __pstl/default.h in C++17 and C++20

The circular include looks like the following:
optional -> __format/range_format.h -> __format/concepts.h -> __format/format_parse_context.h -> string_view -> algorithm -> __algorithm/pstl.h -> __pstl/backend.h -> __pstl/backends/default.h -> optional

Author: smallp-o-p

libcxx/include/optional

@@ -201,7 +201,6 @@ namespace std {
 #  include <__config>
 #  include <__cstddef/ptrdiff_t.h>
 #  include <__exception/exception.h>
-#  include <__format/range_format.h>
 #  include <__functional/hash.h>
 #  include <__functional/invoke.h>
 #  include <__functional/unary_function.h>
@@ -250,6 +249,10 @@ namespace std {
 #  include <initializer_list>
 #  include <version>
 
+#  if _LIBCPP_STD_VER >= 26
+#    include <__format/range_format.h>
+#  endif
+
 // standard-mandated includes
 
 // [optional.syn]