sout: Don't wait until all the es has atleast minimum amount of blocks in fifo

So if one elemental stream stops inputing data to muxer, it can still mux
other tracks.

Ref #4520

src/stream_output/stream_output.c

@@ -556,14 +556,7 @@ int sout_MuxGetStream( sout_mux_t *p_mux, int i_blocks, mtime_t *pi_dts )
         block_t *p_data;
 
         if( block_FifoCount( p_input->p_fifo ) < i_blocks )
-        {
-            if( p_input->p_fmt->i_cat != SPU_ES )
-            {
-                return -1;
-            }
-            /* FIXME: SPU muxing */
             continue;
-        }
 
         p_data = block_FifoShow( p_input->p_fifo );
         if( i_stream < 0 || p_data->i_dts < i_dts )