iwlwifi: remove deprecated and unused iwl_mvm_keyinfo struct

This struct hasn't been used in years and is just a remnant of an API
support removal that missed this structured.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200418110539.93860da2d12a.Ifeca3b3313e3f14330317bc3e3d62f7d991ec955@changeid