avcodec/avcodec: Don't mention removed function

avcodec_thread_init() has been removed in
9a79bb552a.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/avcodec.h

@@ -1518,7 +1518,6 @@ typedef struct AVCodecContext {
      * It will return only after finishing all tasks.
      * The user may replace this with some multithreaded implementation,
      * the default implementation will execute the parts serially.
-     * Also see avcodec_thread_init and e.g. the --enable-pthread configure option.
      * @param c context passed also to func
      * @param count the number of things to execute
      * @param arg2 argument passed unchanged to func