Delete SECURITY.md from v5.6

It's too easily out of date in the stable branches.
It's not included in the release packages anyway.

.github/SECURITY.md

@@ -1,29 +0,0 @@
-# Security Policy
-
-## Supported Versions
-
-We provide security updates to the development branch and the stable
-branches. Security patches for old releases are available on the
-[project website](https://xz.tukaani.org/xz-utils/).
-
-## Reporting a Vulnerability
-
-If you discover a security vulnerability in this project, please
-report it privately. **Do not disclose it as a public issue.** This gives
-us time to work with you to fix the issue before public exposure, reducing
-the chance that the exploit will be used before a patch is released.
-
-You may submit a report by emailing us at
-[xz@tukaani.org](mailto:xz@tukaani.org), or through
-[Security Advisories](https://github.com/tukaani-project/xz/security/advisories/new).
-While both options are available, we prefer email. In any case, please
-provide a clear description of the vulnerability including:
-
-- Affected versions of XZ Utils
-- Estimated severity (low, moderate, high, critical)
-- Steps to recreate the vulnerability
-- All relevant files (core dumps, build logs, input files, etc.)
-
-This project is maintained by a team of volunteers on a reasonable-effort
-basis. As such, please give us 90 days to work on a fix before
-public exposure.