Remove incorrect return statement from avcodec_thread_free()

The function return type is void, so a return statement with an
expression is forbidden (and pointless).
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit b4668274)

libavcodec/utils.c

@@ -1340,7 +1340,7 @@ int avcodec_thread_init(AVCodecContext *s, int thread_count)
 void avcodec_thread_free(AVCodecContext *s)
 {
 #if HAVE_THREADS
-    return ff_thread_free(s);
+    ff_thread_free(s);
 #endif
 }