QAtomicScopedValueRollback: remove impossible constexpr

Neither atomic::load() nor store() are contstexpr in the standard. The
only reason this code compiled is because QASVR is a class template,
so contexpr tags that can't be fulfilled are silently dropped at
instantiation time by the compiler.

Remove the tag to avoid giving readers the wrong idea.

Reported-by: Thiago Macieira <thiago.macieira@intel.com>
Change-Id: I6f23ab9fb2e8ceecaa45ffaef1fadbc7de266803
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit c1f1523047b2e005fb7442847c2aa000faa7d0ac)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qatomicscopedvaluerollback.h

@@ -104,15 +104,12 @@ public:
                                std::memory_order mo = std::memory_order_seq_cst)
         : QAtomicScopedValueRollback(var._q_value, value, mo) {}
 
-#if __cpp_constexpr >= 201907L
-    constexpr
-#endif
     ~QAtomicScopedValueRollback()
     {
         m_atomic.store(m_value, store_part(m_mo));
     }
 
-    constexpr void commit()
+    void commit()
     {
         m_value = m_atomic.load(load_part(m_mo));
     }