pwm: Remove outdated documentation for pwmchip_remove()

pwmchip_remove() returns void since some time but the documentation still
mentions the situations where it used to return an error code. Just remove
this old and now wrong text.

Fixes: 8083f58d ("pwm: Make pwmchip_remove() return void")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>