iwlagn: remove old EEPROM TX power reading

This removes the old TX power reading code, it isn't
necessary since the new code is able to read all the
various EEPROM layouts due to relying on information
contained in the EEPROM.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>