Use var8x8 instead of get8x8var in VP8_UVSSE

'sum' returned by get8x8var is not used and var8x8 has optimizations
  for more platforms.

Change-Id: I4a907fb1a05f285669fb0b95dc71d42182c980f6

vp8/encoder/rdopt.c

@@ -464,11 +464,10 @@ int VP8_UVSSE(MACROBLOCK *x, const vp8_variance_rtcd_vtable_t *rtcd)
     }
     else
     {
-        int sum2, sum1;
-        VARIANCE_INVOKE(rtcd, get8x8var)(uptr, pre_stride,
-            upred_ptr, uv_stride, &sse2, &sum2);
-        VARIANCE_INVOKE(rtcd, get8x8var)(vptr, pre_stride,
-            vpred_ptr, uv_stride, &sse1, &sum1);
+        VARIANCE_INVOKE(rtcd, var8x8)(uptr, pre_stride,
+            upred_ptr, uv_stride, &sse2);
+        VARIANCE_INVOKE(rtcd, var8x8)(vptr, pre_stride,
+            vpred_ptr, uv_stride, &sse1);
         sse2 += sse1;
     }
     return sse2;