Remove CHROMECAST_BUILD from chromecast/

I noticed this used by mistake in a couple CLs recently. The define
isn't set in gn, misleadingly makes people think their code is
tested on cast_shell bot.

TBR=slan@chromium.org

Bug: None
Test: None
Change-Id: Ie4b16ca4b1bb565fe8d45598f7cc0601277907d9
Reviewed-on: https://chromium-review.googlesource.com/1139214
Commit-Queue: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576276}

chromecast/renderer/cast_content_renderer_client.h

@@ -16,10 +16,6 @@
 #include "media/base/audio_codecs.h"
 #include "mojo/public/cpp/bindings/binding.h"
 
-#if defined(CHROMECAST_BUILD)
-#include <string>
-#endif
-
 namespace extensions {
 class ExtensionsClient;
 class ExtensionsGuestViewContainerDispatcher;