Return 0 for wxSYS_CARET_ON_MSEC in wxGTK if appropriate too

For some reason, 7f10d1fa8a handled
wxSYS_CARET_ON_MSEC and wxSYS_CARET_OFF_MSEC differently in wxGTK, even
though it looks like they should be handled in exactly the same way.

Change the code to do this, which results in returning 0 for the former
setting if the caret doesn't blink at all, just as was already the case
for the latter one.

See https://github.com/wxWidgets/wxWidgets/pull/1285

See #17629.

src/gtk/settings.cpp

@@ -975,16 +975,6 @@ int wxSystemSettingsNative::GetMetric( wxSystemMetric index, wxWindow* win )
             return dclick;
 
         case wxSYS_CARET_ON_MSEC:
-            {
-                gint blink_time = -1;
-                g_object_get(GetSettingsForWindowScreen(window),
-                                "gtk-cursor-blink-time", &blink_time, NULL);
-                if (blink_time > 0)
-                    return blink_time / 2;
-
-                return -1;
-            }
-
         case wxSYS_CARET_OFF_MSEC:
             {
                 gboolean should_blink = true;