iwlwifi: remove IWLWIFI_DEBUG_EXPERIMENTAL_UCODE

This Kconfig option allows to load a firmware for
debugging with a different name. This mechanism has not
been used for a few years now and replacing the firmware
file works as well.
Kill this Kconfig option and all the code that goes with it.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>