iwlwifi: remove watchdog debugfs file

This file isn't really all that useful as
when the watchdog triggered it's already
too late, and the setting doesn't persist
unlike e.g. a module parameter that could
be added to the right config file.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>