Remove unused header in aw_contents_io_thread_client_impl.cc

BUG=none

Review URL: https://codereview.chromium.org/1641533003

Cr-Commit-Position: refs/heads/master@{#371817}

android_webview/native/aw_contents_io_thread_client_impl.cc

@@ -14,7 +14,6 @@
 #include "base/android/jni_string.h"
 #include "base/android/jni_weak_ref.h"
 #include "base/lazy_instance.h"
-#include "base/memory/linked_ptr.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/synchronization/lock.h"
 #include "content/public/browser/browser_thread.h"