ARM: at91/pm: remove unused void (*at91_pm_standby)(void)

Because the standby mode use the same sram function as the suspend to memory
mode, void (*at91_pm_standby)(void) doesn't need, remove it.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Tested-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>