iwlwifi: remove rs_get_rate workaround

This removes the work around implemented for transmitting on
an unsupported band on iwlwifi. This was added via the patch:

8e1856e82cb8f541e925738bebfbc473420cda68:
iwlwifi: fix rs_get_rate WARN_ON()

Cc: Mohamed Abbas <mohamed.abbas@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>