iwlwifi: mvm: fix firmware statistics usage

The new (CDB) statistics API is used by non-CDB devices
as well. Look at the right TLV flag to know which version
of the statistics notification to use.
To avoid confusion, remove the _cdb suffix from the
structure name.
While at it, remove a structure that was never used.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Fixes: 678d9b6d ("iwlwifi: mvm: update rx statistics cmd api")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>