Remove unused GetTypeSpecificIdCB callback.

This is part of the base::Callback migration.

Context: https://cs.chromium.org/chromium/src/docs/callback.md?rcl=9fcc3764aea8f97e9f6de4a9ee61d554e67edcda&l=40

Bug: 714018
Change-Id: I25bbcdd96b395a9e5b2fe90c5b509f383845327d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2070237
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Auto-Submit: Jose Lopes <jabolopes@google.com>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744312}

media/mojo/clients/mojo_renderer_factory.h

@@ -32,8 +32,6 @@ class MojoRenderer;
 // MediaPlayerRendererClientFactory for examples of small wrappers around MRF.
 class MojoRendererFactory : public RendererFactory {
  public:
-  using GetTypeSpecificIdCB = base::Callback<std::string()>;
-
   explicit MojoRendererFactory(
       media::mojom::InterfaceFactory* interface_factory);
   ~MojoRendererFactory() final;