mailbox: zynqmp-ipi: drop irq_to_desc() call

irq_to_desc() is not exported to loadable modules, so this driver now
fails to link in some configurations:

ERROR: modpost: "irq_to_desc" [drivers/mailbox/zynqmp-ipi-mailbox.ko] undefined!

I can't see a purpose for this call, since the return value is unused
and probably left over from some code refactoring.

Address the link failure by just removing the line.

Fixes: 6ffb1635 ("mailbox: zynqmp: handle SGI for shared IPI")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Tanmay Shah <tanmay.shah@amd.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>