iwlwifi: mvm: remove unused .remove_sta_debugfs callback

The .remove_sta_debugfs callback was not doing anything in this driver,
so remove it as it is not needed.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20190612142658.12792-4-gregkh@linuxfoundation.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>