avcodec/avrndec: Remove obsolete FIXME

This FIXME probably meant to suggest to use a AVFrame that no longer
exists in the context.

Found-by: durandal117
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avrndec.c

@@ -28,7 +28,7 @@
 typedef struct {
     AVCodecContext *mjpeg_avctx;
     int is_mjpeg;
-    int interlace; //FIXME use frame.interlaced_frame
+    int interlace;
     int tff;
 } AVRnContext;