Remove unused variables, fixes the warnings:

libavcodec/wmadec.c:629: warning: unused variable `i'
libavcodec/wmadec.c:628: warning: unused variable `b'
libavcodec/wmadec.c:628: warning: unused variable `a'

Originally committed as revision 19994 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/wmadec.c

@@ -625,9 +625,6 @@ static int wma_decode_block(WMACodecContext *s)
 #endif
 
     if (s->ms_stereo && s->channel_coded[1]) {
-        float a, b;
-        int i;
-
         /* nominal case for ms stereo: we do it before mdct */
         /* no need to optimize this case because it should almost
            never happen */