Remove unused assignment from j_rev_dct() found by CSA.

Originally committed as revision 18558 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/jrevdct.c

@@ -716,7 +716,6 @@ void j_rev_dct(DCTBLOCK data)
                 if (d1) {
                     /* d1 != 0, d3 == 0, d5 != 0, d7 != 0 */
                     z1 = d7 + d1;
-                    z2 = d5;
                     z3 = d7;
                     z4 = d5 + d1;
                     z5 = MULTIPLY(z3 + z4, FIX_1_175875602);