ARM: at91: remove useless at91_sysirq_mask_rtx

The irq fixup from at91_sysirq_mask_rtc and at91_sysirq_mask_rtt is now handled
by aic_common_rtc_irq_fixup and aic_common_rtt_irq_fixup. Remove those useless
functions.
Also remove the now unused mach/at91_rtt.h header.

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