Remove unnecessary memset for lowbd case

Errors reported in BUG=aomedia:1363 only show up for highbd case.
The memset in lowbd branch is unnecessary, and caused encoder
about 8% slower, showed by a simple test with default config
encode 20 frames of foreman_cif.y4m.

Change-Id: Ic19c592acf5cc4beb0390e08c7133e4a0ce1a54f

av1/encoder/mcomp.c

diff --git a/av1/encoder/mcomp.c b/av1/encoder/mcomp.c
index 7bdfb02..f3dcc70 100644
--- a/av1/encoder/mcomp.c
+++ b/av1/encoder/mcomp.c
@@ -693,7 +693,7 @@
 
     besterr = vfp->vf(CONVERT_TO_BYTEPTR(pred16), w, src, src_stride, sse);
   } else {
-    DECLARE_ALIGNED(16, uint8_t, pred[MAX_SB_SQUARE]) = { 0 };
+    DECLARE_ALIGNED(16, uint8_t, pred[MAX_SB_SQUARE]);
     if (second_pred != NULL) {
       if (mask) {
         aom_comp_mask_upsampled_pred(pred, second_pred, w, h, subpel_x_q3,