ARM: at91/pm_slowclock: remove the unused code related with SLOWDOWN_MASTER_CLOCK

The SLOWDOWN_MASTER_CLOCK definition is not used, remove the redundant code.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>