ARM: at91: remove useless at91_pwm_leds()

Now that all at91 boards using leds-atmel-pwm switched to leds-pwm, the
at91_pwm_leds() function is not used anymore.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>