package/mender: remove tenant.conf

Tenant Token is a configuration option that has to do with Hosted Mender,
where you you need to set this for the devices to connect to the
correct organization in a multi-tenant system.

The removal of tenant.conf usage (and /var/lib/mender/authtentoken)
was in Mender client version 1.2.0, where it was switched to be an mender.conf
option instead as the example above demonstrates. As the first version that was
integrated in Buildroot was 1.4.0, the inclusion of tenant.conf and the
creation of the symlink is not necessary.

Now it is specified as such in mender.conf:

Example:

/etc/mender/mender.conf
{
    TenantToken: "very long base64 encoded string"
}

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>