ARM: at91: remove legacy IRQ driver and related code

Remove irc.c and associated header file. The related code was idendified by
the CONFIG_OLD_IRQ_AT91 option that was removed previously. It has been spotted
by following coccinelle semantic match:

@rule1@
expression E;
statement S;
@@
(
- if (IS_ENABLED(CONFIG_OLD_IRQ_AT91)) S
|
- if (IS_ENABLED(CONFIG_OLD_IRQ_AT91) && E) S
)

Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>