pwm: Remove unused radix tree

The radix tree's only use was to map PWM channels to the global number
space. With that number space gone, the radix tree is now unused, so it
can simply be removed.

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>