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 <[email protected]>
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff