Revert "avcodec/cbs_h264: silence errors about end_of_seq nalus"

This reverts commit aec3daa8.

This is an incomplete fix, and did not quality for a backport.

libavcodec/cbs_h2645.c

@@ -828,9 +828,6 @@ static int cbs_h264_read_nal_unit(CodedBitstreamContext *ctx,
         }
         break;
 
-    case H264_NAL_END_SEQUENCE:
-        return 0;
-
     default:
         return AVERROR(ENOSYS);
     }