watchdog: omap_wdt: eliminate unused variable and a compiler warning

We forgot to delete this in the commit 4f4753d9 (watchdog: omap_wdt:
convert to devm_ functions), and as a result the following compilation
warning was introduced:

drivers/watchdog/omap_wdt.c: In function 'omap_wdt_remove':
drivers/watchdog/omap_wdt.c:299:19: warning: unused variable 'res' [-Wunused-variable]

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>