Avcodec, restore the threads count to auto

Mpeg-4, H.264, VP8 and other deadlocks with -mt have been fixed
and fuzzed by Janne, notably, since a few weeks/months.

modules/codec/avcodec/avcodec.c

@@ -139,7 +139,7 @@ vlc_module_begin ()
     add_module( "avcodec-hw", "hw decoder", "none", HW_TEXT, HW_LONGTEXT, false )
 #if defined(FF_THREAD_FRAME)
     add_obsolete_integer( "ffmpeg-threads" ) /* removed since 2.1.0 */
-    add_integer( "avcodec-threads", 1, THREADS_TEXT, THREADS_LONGTEXT, true );
+    add_integer( "avcodec-threads", 0, THREADS_TEXT, THREADS_LONGTEXT, true );
 #endif