contrib: drop hg-to-git script

The hg-to-git script is full of command injection vulnerabilities
against malicious branch and tag names. It's also old and largely
unmaintained; the last commit was over 4 years ago, and the last code
change before that was from 2013. Users are better off with a modern
remote-helper tool like cinnabar or remote-hg.

So rather than spending time to fix it, let's just get rid of it.

Reported-by: Matthew Rollings <admin@stealthcopter.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff