compat: remove an unused symbol

The compatibility issue actually didn#t get solved, it's just handled
differently in mpv now.

compat/libav.h

@@ -28,8 +28,4 @@
 #define AV_CPU_FLAG_MMX2 AV_CPU_FLAG_MMXEXT
 #endif
 
-#if (LIBAVUTIL_VERSION_MICRO < 100) || (LIBAVCODEC_VERSION_INT < AV_VERSION_INT(54, 53, 100))
-#define AV_CODEC_ID_SUBRIP AV_CODEC_ID_TEXT
-#endif
-
 #endif /* MPV_LIBAV_COMPAT_H */