qxp::function_ref: simplify the constraint on op=(T) = delete

Drop the remove_cvref_t (T is already decayed by way of take-by-value)
and apply De Morgan's Law.

No functional change, just fewer templates to instantiate.

Amends 29b65c98e720056e87334ce88a683969e57efd3d.

Pick-to: 6.5
Change-Id: I0c0f8c4a288145d05636fb053cc260a917fcc4c5
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 0d0cd1fe2c4f2d90a8184ce88383bdf1ff35b338)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 0f675e0f62d03aeb3d6718147954b0368d955c71)

src/corelib/global/qxpfunctional.h

@@ -141,9 +141,11 @@ public:
 protected:
     template <
         class T,
-        std::enable_if_t<std::conjunction_v<
-            std::negation<std::is_same<q20::remove_cvref_t<T>, function_ref_base>>,
-            std::negation<std::is_pointer<T>>
+        std::enable_if_t<std::negation_v<
+            std::disjunction<
+                std::is_same<T, function_ref_base>,
+                std::is_pointer<T>
+            >
         >, bool> = true
     >
     function_ref_base& operator=(T) = delete;