MojoVideoEncodeAcceleratorService: Delete one outdated TODO

This CL just deletes one TODO. It stated that we would call
VEA::TryToSetupEncodeOnSeparateThread() with an ad-hoc background worker thread.
In our current stack, there is no need to do this.

BUG=chromium:828047
TEST=None

Change-Id: I034e6a9e20101e54b08a664479bffb4e8f3df0f9
Reviewed-on: https://chromium-review.googlesource.com/997054Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548404}

media/mojo/services/mojo_video_encode_accelerator_service.cc

@@ -84,10 +84,6 @@ void MojoVideoEncodeAcceleratorService::Initialize(
     return;
   }
 
-  // TODO(mcasas): We could still TryToSetupEncodeOnSeparateThread() with an
-  // ad-hoc background worker thread, but for the time being this doesn't seem
-  // necessary since we're already on a background thread.
-
   std::move(success_callback).Run(true);
   return;
 }