iwlwifi: mvm: MVM op_mode is supported on 7000 only

The code removed in this patch was used for bring up on
older NICs. No MVM capable fw will ever be released for
older NICs, so remove that code.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>