libobs: Deprecate service multitrack check

This isn't particularly needed, as a service with multiple tracks won't
be using multiple tracks to begin with anyway.  This might change later,
but for now just mark it deprecated.

plugins/rtmp-services/rtmp-common.c

@@ -645,5 +645,4 @@ struct obs_service_info rtmp_common_service = {
 	.get_key = rtmp_common_key,
 	.apply_encoder_settings = rtmp_common_apply_settings,
 	.get_output_type = rtmp_common_get_output_type,
-	.supports_multitrack = supports_multitrack,
 };