Remove Q_DUMMY_COMPARISON_OPERATOR

It's not needed for any of our supported compilers and isn't used in
Qt codbase anymore. It was only required for IBM xlC compiler, which
doesn't seem to support C++17 at the moment (when it does, hopefully
they will also fix the issue).

[ChangeLog][QtGlobal] Removed the Q_DUMMY_COMPARISON_OPERATOR macro,
which was needed to workaround outdated template instantiation rules
on some outdated compilers.

Fixes: QTBUG-105098
Change-Id: I2b992dd8c0f389d874d2a8521fc1a1aad3f80699
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/global/qcompilerdetection.h

@@ -223,7 +223,6 @@
               but it is not defined on older compilers like C Set 3.1 */
 #elif defined(__xlC__)
 #  define Q_CC_XLC
-#  define Q_FULL_TEMPLATE_INSTANTIATION
 #  if __xlC__ < 0x400
 #    error "Compiler not supported"
 #  elif __xlC__ >= 0x0600

src/corelib/global/qtclasshelpermacros.h

@@ -100,26 +100,6 @@ template <typename Ptr> inline auto qGetPtrHelper(Ptr &ptr) noexcept -> decltype
 #define Q_D(Class) Class##Private * const d = d_func()
 #define Q_Q(Class) Class * const q = q_func()
 
-/*
-   Compilers which follow outdated template instantiation rules
-   require a class to have a comparison operator to exist when
-   a QList of this type is instantiated. It's not actually
-   used in the list, though. Hence the dummy implementation.
-   Just in case other code relies on it we better trigger a warning
-   mandating a real implementation.
-*/
-
-#ifdef Q_FULL_TEMPLATE_INSTANTIATION
-#  define Q_DUMMY_COMPARISON_OPERATOR(C) \
-    bool operator==(const C&) const { \
-        qWarning(#C"::operator==(const "#C"&) was called"); \
-        return false; \
-    }
-#else
-
-#  define Q_DUMMY_COMPARISON_OPERATOR(C)
-#endif
-
 /*
    Specialize a shared type with: