Remove incorrect comment; MMX2 is preferred over 3DNow! on Athlon

Originally committed as revision 9079 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/i386/dsputil_mmx.c

@@ -177,7 +177,6 @@ static const uint64_t ff_pb_FC attribute_used __attribute__ ((aligned(8))) = 0xF
 /* 3Dnow specific */
 
 #define DEF(x) x ## _3dnow
-/* for Athlons PAVGUSB is preferred */
 #define PAVGB "pavgusb"
 
 #include "dsputil_mmx_avg.h"