The cursor is referenced during CopyGrab(), thus doesn't need to be handled
manually anymore. It does need to be refcounted for temp grabs though.

The oldGrab handling in ProcGrabPointer is a leftover from the cursor in the
grab being refcounted, but the grab itself being a static struct in the
DeviceIntRec. Now that all grabs are copied, this lead to a double-free of
the cursor (Reproduced in Thunderbird, dragging an email twice (or more
often) causes a crash).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>