iwlwifi: remove antenna_coupling module parameter

This module parameter should not be mangled by users.
This relates to a very old driver and I doubt people can
really check the antenna coupling in a way that would make
the BT Coexistence work better with a real value.
Drop it.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200424182644.6e566897ce0a.I8395a50c1c39522e542366064bff33a33009ce7b@changeid