Fix compile error when both of dist_8x8 and pvq are enabled

Remove mistakenly leftover code from commit
e30a47ca0ff42ca16561ff6ba2099a4233a5bc75.

Note that this change force-disables activity masking in PVQ until we can add some
bitstream signaling for it.

Change-Id: I5279f82e7977733ebc40f5aed844ecec1279992a

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 1b847ba..a1b06a2 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -3739,9 +3739,6 @@
     uint8_t *src = p->src.buf;
     uint8_t *dst = pd->dst.buf;
 
-#if CONFIG_PVQ
-    use_activity_masking = mb->daala_enc.use_activity_masking;
-#endif  // CONFIG_PVQ
     // Daala-defined distortion computed for the block of 8x8 pixels
     total_distortion = av1_dist_8x8(cpi, mb, src, src_stride, dst, dst_stride,
                                     BLOCK_8X8, 8, 8, 8, 8, mb->qindex)