mpm_winnt: remove the AcceptEx data network bucket

Follow-up to the prior commit: without an incoming data buffer, the
custom network bucket code is now orphaned and we can remove it
entirely. This has the added benefit that we are no longer using the
internal OVERLAPPED.Pointer field, which is discouraged by the MSDN
docs.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1758308 13f79535-47bb-0310-9956-ffa450edef68