files: Kill file_desc_ops::want_transport()

Since transport fd is per-task, this method is not need anymore.
Kill it.

travis-ci: success for Rework file opening scheme to make it asynchronous (rev5)
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>