Remove unused NetPrefObserver forward declaration.

This CL removes the last reference of NetPrefObserver from the codebase.  This
is a follow-up to https://crrev.com/2172543003.

Review-Url: https://codereview.chromium.org/2179403003
Cr-Commit-Position: refs/heads/master@{#408208}

chrome/browser/profiles/profile_impl.h

@@ -23,7 +23,6 @@
 #include "content/public/browser/content_browser_client.h"
 #include "content/public/browser/host_zoom_map.h"
 
-class NetPrefObserver;
 class PrefService;
 
 class TrackedPreferenceValidationDelegate;