Delete less-than operator for QKeyCombination

QMetaType relies on QTypeTraits to identify whether a type provides
operator<. Due to the the deprecated implicit conversion to int, this
results in several screens of deprecation warnings when using
QKeyCombination in a signal parameter or as a property type.

QKeyCombination has no ordering semantics (is Alt+F5 larger than
Control+Up?), so delete the operator.

[ChangeLog][QtCore][QKeyCombination] Potentially source-incompatible change:
the less-than operator for QKeyCombination has been deleted. Code relying
on the deprecated implicit conversion to int will no longer compile.
Use QKeyCombination::toCombined explicitly.

Change-Id: Ic8522b88d68063f74b5a5582c51aee8e18982331
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/global/qnamespace.h

@@ -1919,6 +1919,8 @@ public:
     {
         return lhs.combination != rhs.combination;
     }
+
+    bool operator<(QKeyCombination) const = delete;
 };
 
 Q_DECLARE_TYPEINFO(QKeyCombination, Q_RELOCATABLE_TYPE);