iwlwifi: mvm: rs: remove max_rate_idx

max_rate_idx constraint is deprecated and it's handling is
faulty as well as it is relevant only for legacy rates but
was considered in HT/VHT. In most cases there was no side effect
as max_rate_idx was set to -1 but in certain cases like P2P
it got set to an actual rate idx which would limit the maximum
rate in HT/VHT by mistake.
max_rate_idx should be replaced by the masks fields but for
now remove it.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>