bmx6: drop package

This package is no longer maintained by us or by upstream developers.
The last version in the GitHub repository is from 2020 with no activity so far.
We are using version 0.1-alpha.

Because LuCI app is vulnerable to several CVEs and DependencyBot still
sends emails about it, let's drop it.

If anyone wants, they can use bmx7.

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