Remove a temporary assertion.

The assertion was added a month ago in
commit c02bc634e5a26a73352932a5d24f374a9ae71424:
https://aomedia-review.googlesource.com/c/aom/+/71792

The assertion has not failed in the past month, so it is safe to remove
it now.

BUG=oss-fuzz:10389

Change-Id: Ife9d97cc4b9de4f154abdd58149d1bfa446739d2
diff --git a/av1/decoder/decoder.c b/av1/decoder/decoder.c
index ba13741..c0b55b6 100644
--- a/av1/decoder/decoder.c
+++ b/av1/decoder/decoder.c
@@ -343,8 +343,6 @@
     }
 
     // Current thread releases the holding of reference frame.
-    // TODO(wtc): Remove this assertion after 2018-10-31.
-    assert(!cm->show_existing_frame || cm->reset_decoder_state);
     for (; ref_index < REF_FRAMES; ++ref_index) {
       const int old_idx = cm->ref_frame_map[ref_index];
       decrease_ref_count(old_idx, frame_bufs, pool);