Haiku: Use 'unknown' as QSysInfo::productType()

There are no actual products of Haiku, so using
'unknown' (the default) is fine, otherwise the
file platform selector would be '+unix/+haiku/+haiku'
instead of '+unix/+haiku'.

Change-Id: Id7653098e20374885a50c09e2aaac9e6fcfc6efb
Reviewed-by: Augustin Cavalier <waddlesplash@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.cpp

@@ -2529,9 +2529,6 @@ QString QSysInfo::productType()
 #elif defined(Q_OS_DARWIN)
     return QStringLiteral("darwin");
 
-#elif defined(Q_OS_HAIKU)
-    return QStringLiteral("haiku");
-
 #elif defined(USE_ETC_OS_RELEASE) // Q_OS_UNIX
     QUnixOSVersion unixOsVersion;
     readEtcOsRelease(unixOsVersion);