iwlwifi: mvm: BT Coex - update integration with rate control

Remove code that is not needed and always allow MIMO when
in tight mode. In loose mode, we should avoid MIMO since BT
can use the other antenna to Rx while we Tx.

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