luci-app-bmx6: drop this because of security vulnerabilities

DependencyBot, which are using is sending us emails about these CVEs:
CVE-2012-6708
CVE-2020-23064
CVE-2019-11358

This was reported to maintainer in April 2023, but no one stepped it to fix that,
so let's drop this.

Replacement could be luci-app-bmx7.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>