cgroupfs-mount: remove package

cgroupfs-mount: remove package

A user opened an issue[1] in the upstream repository to migrate
the package from GitHub to Debian Salsa, but the upstream response[2]
was as follows:

> If any work happens on this package, it should be to deprecate and remove
> it, not migrate it (that's a waste of effort).

[1] https://github.com/tianon/cgroupfs-mount/issues/24
[2] https://github.com/tianon/cgroupfs-mount/issues/24#issuecomment-2663677115

Signed-off-by: Wesley Gimenes <[email protected]>