pwm: Drop per-chip dbg_show callback

This callback was introduced in commit 62099abf

 ("pwm: Add debugfs
interface") in 2012 and up to now there is not a single user. So drop
this unused code.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[thierry.reding@gmail.com: remove kerneldoc for ->dbg_show()]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>