Remove read_payload conceptual requirement

Author: mborland

doc/modules/ROOT/pages/cmath.adoc

@@ -550,7 +550,7 @@ Effects: If x is an sNaN returns true, otherwise returns false.
 ----
 template <typename Decimal>
 constexpr auto read_payload(Decimal x) noexcept
-    -> std::enable_if_t<is_ieee_type_v<Decimal>, typename T::significand_type>;
+    -> typename T::significand_type;
 ----
 
 Effects: if x is either a qNaN or an sNaN, returns the payload of the NaN, otherwise returns 0.