Remove unreachable not-found branch in dns-export-zone

The command loaded the zone with Zone.objects.get(...) and then tested
the result for truthiness:

    nb_zone = Zone.objects.get(...)
    if nb_zone:
        ...export...
    else:
        self.stderr.write('Zone not found in Netbox. Aborting')

Zone.objects.get() either returns a model instance (always truthy) or
raises Zone.DoesNotExist. It never returns a falsy value, so the else
branch was unreachable dead code. A missing zone is already handled
correctly: DoesNotExist is a subclass of Exception, which the
surrounding try/except catches and converts into a CommandError.

Drop the dead if/else and call export_bind_zone_file directly.

netbox_dns_bridge/management/commands/dns-export-zone.py

@@ -37,13 +37,10 @@ class Command(BaseCommand):
                 view__name=view_name, name=zone_name
             )
 
-            if nb_zone:
             export_bind_zone_file(nb_zone, file_path=file_path)
             self.stdout.write(
                 self.style.SUCCESS(f"Zone '{zone_name}' exported to '{file_path}'")
             )
-            else:
-                self.stderr.write("Zone not found in Netbox. Aborting")
 
         except Exception as e:
             raise CommandError(f"Failed to export zone: {e}")