h264: remove an artificial restriction on the number of slice threads

This limit is now unnecessary, we can easily support an arbitrary number
of threads.

libavcodec/h264.h

@@ -49,7 +49,6 @@
 #include "videodsp.h"
 
 #define H264_MAX_PICTURE_COUNT 32
-#define H264_MAX_THREADS       16
 
 #define MAX_SPS_COUNT          32
 #define MAX_PPS_COUNT         256
@@ -593,8 +592,6 @@ typedef struct H264Context {
      */
     int max_contexts;
 
-    int slice_context_count;
-
     /** @} */
 
     /**