d/postinst: actively remove pvetest repository (added for beta) again

d/postinst: actively remove pvetest repository (added for beta) again

in theory we'd need to be more cautios but this was added only during
beta, which is when we do not really provided any stability
guarantee, further, it's rather unlikely that one added very
important repos that, when removed, break something (again *during*
beta).

The new APT repo management makes it also easy to see when one does
not gets any PVE updates, and one can add the pvetest repo there
again easily too.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>