Remove unneeded type casts.

This is a follow-up to https://aomedia-review.googlesource.com/c/aom/+/58684.

Also remove the unused static function get_obu_length_field_size() in
av1/av1_dx_iface.c.

Change-Id: I9e01d59c5d449a1748e035d28d1b206fa5f41942
diff --git a/test/coding_path_sync.cc b/test/coding_path_sync.cc
index 0a4694f..8171cf1 100644
--- a/test/coding_path_sync.cc
+++ b/test/coding_path_sync.cc
@@ -152,7 +152,7 @@
 
   std::vector<int16_t> decode(const aom_codec_cx_pkt_t *pkt) {
     aom_codec_decode(&dec_, static_cast<uint8_t *>(pkt->data.frame.buf),
-                     static_cast<unsigned int>(pkt->data.frame.sz), NULL);
+                     pkt->data.frame.sz, NULL);
 
     aom_codec_iter_t iter = NULL;
     return Serialize(aom_codec_get_frame(&dec_, &iter));