Remove FileUtilProxy:DeleteFile().

FileUtilProxy::DeleteFile() deletes a file on a TaskRunner and then
invokes a callback on the source thread. All call sites provide a null
callback. They might as well just post a base::DeleteFile() task to
the TaskRunner.

This is a prerequisite to stop allowing null callbacks in
base::PostTaskAndReplyWithResult().

BUG=162712

Review-Url: https://codereview.chromium.org/2528193002
Cr-Commit-Position: refs/heads/master@{#434765}
5 files changed