Moc: remove GCC 9 ban for using QtMocHelpers::stringData()

Whatever issue used to be present in the CI no longer is. That doesn't
mean the problem went away; Ubuntu 20.04 is probably still shipping a
broken compiler with bad codegen, but this isn't the first issue.

Change-Id: I9201d9ecf52f4146bb04fffd1764673c94ee8955
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qtmochelpers.h

@@ -76,10 +76,7 @@ template <uint... Nx> constexpr auto stringData(const char (&...strings)[Nx])
     return result;
 }
 
-#if !defined(Q_CC_GNU_ONLY) || Q_CC_GNU_ONLY >= 1000
-// It looks like there's a bug in GCC 9
 #  define QT_MOC_HAS_STRINGDATA       1
-#endif
 
 } // namespace QtMocHelpers
 QT_END_NAMESPACE