Removed some unused variables in Windows code.

Note: Regarding the ICMP6_CLOSE_FUNC variable in winping.cc,
Icmp6CloseHandle does not exist, and IcmpCloseHandle is the correct way
to close an IPv6 handle. Therefore the existing code is correct to use
close_ on both types of connections and this variable is unnecessary.

BUG=505319

Review URL: https://codereview.webrtc.org/1231653003

Cr-Commit-Position: refs/heads/master@{#9555}