Remove forward declaration of a class that doesn't exist.

The class NavigationPostDataHandle doesn't exist anymore. It is not
clear to me if it has ever existed.

Bug: None.
Change-Id: I98d95f02b6e3b03f9f31b6ceeac2e472846dd524
Reviewed-on: https://chromium-review.googlesource.com/1039487Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555407}

content/browser/loader/navigation_url_loader_network_service.h

@@ -22,7 +22,6 @@ struct RedirectInfo;
 namespace content {
 
 class NavigationData;
-class NavigationPostDataHandler;
 class ResourceContext;
 class StoragePartition;
 class NavigationLoaderInterceptor;