libobs: Remove starting_frame_count from obs_output_t

The member variable `starting_frame_count` was set but never read.

libobs/obs-internal.h

@@ -1100,7 +1100,6 @@ struct obs_output {
 
 	uint32_t starting_drawn_count;
 	uint32_t starting_lagged_count;
-	uint32_t starting_frame_count;
 
 	int total_frames;
 

libobs/obs-output.c

@@ -330,10 +330,6 @@ bool obs_output_actual_start(obs_output_t *output)
 	if (output->context.data)
 		success = output->info.start(output->context.data);
 
-	if (success && output->video) {
-		output->starting_frame_count =
-			video_output_get_total_frames(output->video);
-	}
 	if (success) {
 		output->starting_drawn_count = obs->video.total_frames;
 		output->starting_lagged_count = obs->video.lagged_frames;