arch: arc: _rirq_exit: Comment clean-up

We manage IRQs in a quite a different way now since
commit f8d061fa ("arch: arc: add nested interrupt support")
so that comment not only makes no sense but also may fool a reader
as disabling of interrupts happens in the very beginning of
_rirq_exit() but not here.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>