mpegvideo: drop vismv code

It has been broken for over a year without anyone complaining or
noticing, thus proving that nobody ever uses it.

libavcodec/mpegvideo.h

@@ -340,7 +340,6 @@ typedef struct MpegEncContext {
     Picture *last_picture_ptr;     ///< pointer to the previous picture.
     Picture *next_picture_ptr;     ///< pointer to the next picture (for bidir pred)
     Picture *current_picture_ptr;  ///< pointer to the current picture
-    uint8_t *visualization_buffer[3]; ///< temporary buffer vor MV visualization
     int last_dc[3];                ///< last DC values for MPEG1
     int16_t *dc_val_base;
     int16_t *dc_val[3];            ///< used for mpeg4 DC prediction, all 3 arrays must be continuous