samba.postinst: do not populate sambashare from the Ubuntu admin group
This reverts commit fe5cef5014db5b5d6cf55e036583f8f84962e9b2.
The admin group has been phased out in Ubuntu 12.04 [1]:
Up until Ubuntu 11.10, administrator access using the sudo tool was
granted via the "admin" Unix group. In Ubuntu 12.04, administrator
access will be granted via the "sudo" group.
We could add users in the sudo group to sambashare, however I don't
think that's really a good idea:
1. The sudo group isn't likely to go away like admin did, but it's still
an arbitrary group, and sudo does have alternatives (polkit, doas), so
it's not really *guaranteed* to stay forever.
2. There's nothing that removes users from sambashare when they're
removed from the admin/sudo group.
3. Users in the sudo group can add themselves to the sambashare group.
4. This has been broken for almost 10 years, with the problem being noticed
only when the mechanism did something unexpected.