Remove an out-of-date assertion

I added this assertion before:
  assert(filter_weight == 2);
for the alt ref frame itself. With the recent change, this was not
true anymore. Removed it.

Change-Id: I57ea878151596bfd69215d8b5d2e66f395c2fd8f
diff --git a/av1/encoder/temporal_filter.c b/av1/encoder/temporal_filter.c
index 5d6b916..ae94c91 100644
--- a/av1/encoder/temporal_filter.c
+++ b/av1/encoder/temporal_filter.c
@@ -169,7 +169,6 @@
                                        int use_new_temporal_mode) {
   const int modifier = use_new_temporal_mode ? SCALE : filter_weight * 16;
   unsigned int i, j, k = 0;
-  assert(filter_weight == 2);
 
   for (i = 0; i < block_height; i++) {
     for (j = 0; j < block_width; j++) {
@@ -188,7 +187,6 @@
   const int modifier = use_new_temporal_mode ? SCALE : filter_weight * 16;
   const uint16_t *pred = CONVERT_TO_SHORTPTR(pred8);
   unsigned int i, j, k = 0;
-  assert(filter_weight == 2);
 
   for (i = 0; i < block_height; i++) {
     for (j = 0; j < block_width; j++) {