Remove #include of ptr_util.h in proxy_policy_handler.cc

After recent changes, proxy_policy_handler.cc no longer needs to include
base/memory/ptr_util.h. This has been pointed in
https://codereview.chromium.org/2785883003/diff/80001/components/policy/core/browser/proxy_policy_handler.cc#newcode11.
Because that comment happened after landing of the original CL, that follow-up
is done here.

R=bartfab@chromium.org
BUG=697817

Review-Url: https://codereview.chromium.org/2793093003
Cr-Commit-Position: refs/heads/master@{#466911}

components/policy/core/browser/proxy_policy_handler.cc

@@ -8,7 +8,6 @@
 
 #include "base/logging.h"
 #include "base/macros.h"
-#include "base/memory/ptr_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/values.h"
 #include "components/policy/core/browser/configuration_policy_handler.h"