iwlagn: remove spectrum measurement header

This header file isn't used, and if we ever need
these definitions they shouldn't be added to a
driver but rather to the common 802.11 include
file that has all frame definitions. Thus, just
remove it.

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