Remove assert on decoder side

This assert will not be true when frames are lost and
interferes with the tests.

BUG=aomedia:1681

Change-Id: If22e884da41057b03d67cd85a7d10ccdfd52ee13

av1/decoder/decodeframe.c

diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 998f465..653ce96 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -2707,8 +2707,6 @@
 
           cm->ref_frame_map[ref_idx] = buf_idx;
           frame_bufs[buf_idx].cur_frame_offset = frame_offset;
-        } else {
-          assert(frame_offset == frame_bufs[buf_idx].cur_frame_offset);
         }
       }
     }