i2c: smbus: remove outdated references to irq level triggers

IRQ levels are now handled within the IRQ core. Remove the forgotten
references from the documentation.

Fixes: 9b9f2b8b ("i2c: i2c-smbus: Use threaded irq for smbalert")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>