MT#56739 Remove already deprecated preference 'enable_t38'

The 'enable_t38' preference was added long time ago to
provision of phones that support fax services with t38.
We don't support those devices since long time.
Additionally the preference creates confusion with real
subscriber feature of enable t38 for mailtofax/faxtomail
service.

Due to those reason the preference can be removed.

Change-Id: Ib9f4f37354a62ad59ba8c5b63ae476142b36e717

lib/NGCP/Panel/Controller/Device.pm

@@ -1347,7 +1347,6 @@ sub dev_field_config :Chained('/') :PathPart('device/autoprov/config') :Args() {
 
             my %sub_preferences_vars = (
                 display_name               => $sub->username,
-                enable_t38                 => 0,
                 concurrent_max             => 0,
                 concurrent_max_per_account => 0,
                 cc                         => '',