iwlwifi: remove disable_tx_power for device > 4965

After 4965, tx_power_calibration moved from driver to uCode.
remove "disable_tx_power" from debugfs to minimize the confusion.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>