pwm: Delete deprecated functions pwm_request() and pwm_free()

Since commit 5a7fbe45 ("backlight: pwm_bl: Drop support for legacy PWM
probing") the last user of pwm_request() and pwm_free() is gone. So remove
these functions that were deprecated over 10 years ago in commit
8138d2dd ("pwm: Add table-based lookup for static mappings").

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[thierry.reding@gmail.com: clean up a bit after removal]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>