Input: ads7846 - remove custom filter handling functions from pdata

The functions in the platform data struct to initialize, cleanup and
apply custom filters are not in use by any mainline board.

Remove support for them to pave the road for more cleanups to come.

The enum was moved as it has no users outside of the driver code
itself.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Link: https://lore.kernel.org/r/20210907200726.2034962-3-daniel@zonque.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>