lavc: remove FF_API_RATE_EMU cruft

libavcodec/avcodec.h

@@ -1203,16 +1203,6 @@ typedef struct AVCodecContext {
      */
     enum PixelFormat pix_fmt;
 
-#if FF_API_RATE_EMU
-    /**
-     * Frame rate emulation. If not zero, the lower layer (i.e. format handler)
-     * has to read frames at native frame rate.
-     * - encoding: Set by user.
-     * - decoding: unused
-     */
-    attribute_deprecated int rate_emu;
-#endif
-
     /**
      * If non NULL, 'draw_horiz_band' is called by the libavcodec
      * decoder to draw a horizontal band. It improves cache usage. Not