mojo: Remove incorrect DCHECK in MojoDecryptorService

This doesn't align with the media::Decryptor documentation where it
indicates it should return kSuccess and a null buffer if CancelDecrypt
is called.

BUG=None
TEST=Builds

Change-Id: Ib0654014262f61db7875274350d92a7510c065ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570127
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Auto-Submit: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833137}

media/mojo/services/mojo_decryptor_service.cc

@@ -197,7 +197,6 @@ void MojoDecryptorService::OnDecryptDone(DecryptCallback callback,
   DVLOG_IF(3, status == Status::kSuccess) << __func__;
 
   if (!buffer) {
-    DCHECK_NE(status, Status::kSuccess);
     std::move(callback).Run(status, nullptr);
     return;
   }