misc-fix: use subexp prob updates for MV probs

This commit merges the fix that changes to use subexp probability
updates for update MV probabilities.

Related tracking issue:
https://bugs.chromium.org/p/webm/issues/detail?id=1040
Original libvpx commit:
177e7b53e7e88a94b20cf04207803231c7116f9e

Change-Id: Id1c79a507f3ae698c3560a559b7b630974bcf656

av1/decoder/decodeframe.c

diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 546e3df..6bc4eac 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -208,12 +208,7 @@
 static void update_mv_probs(aom_prob *p, int n, aom_reader *r) {
   int i;
   for (i = 0; i < n; ++i)
-#if CONFIG_MISC_FIXES
     av1_diff_update_prob(r, &p[i], ACCT_STR);
-#else
-    if (aom_read(r, MV_UPDATE_PROB, ACCT_STR))
-      p[i] = (aom_read_literal(r, 7, ACCT_STR) << 1) | 1;
-#endif
 }
 
 static void read_mv_probs(nmv_context *ctx, int allow_hp, aom_reader *r) {

av1/encoder/encodemv.c

diff --git a/av1/encoder/encodemv.c b/av1/encoder/encodemv.c
index ce946f4..5667ee0 100644
--- a/av1/encoder/encodemv.c
+++ b/av1/encoder/encodemv.c
@@ -140,7 +140,6 @@
 
 static void update_mv(aom_writer *w, const unsigned int ct[2], aom_prob *cur_p,
                       aom_prob upd_p) {
-#if CONFIG_MISC_FIXES
   (void)upd_p;
 #if CONFIG_TILE_GROUPS
   // Just use the maximum number of tile groups to avoid passing in the actual
@@ -149,16 +148,6 @@
 #else
   av1_cond_prob_diff_update(w, cur_p, ct, 1);
 #endif
-#else
-  const aom_prob new_p = get_binary_prob(ct[0], ct[1]) | 1;
-  const int update = cost_branch256(ct, *cur_p) + av1_cost_zero(upd_p) >
-                     cost_branch256(ct, new_p) + av1_cost_one(upd_p) + 7 * 256;
-  aom_write(w, update, upd_p);
-  if (update) {
-    *cur_p = new_p;
-    aom_write_literal(w, new_p >> 1, 7);
-  }
-#endif
 }
 
 static void write_mv_update(const aom_tree_index *tree,