Removing the ast account

Removing the ast account

We used to have an account "ast" as a token to Andrew S. Tannebaum,
the creator of MINIX. The account was disable by default, with no
possible way to login.

It seems it generates more confusion than it is worth, so this removes it.

The default skeleton user profile is still stored into /etc/ast instead
of the usual /etc/skel.

Change-Id: I71818c0bb9580cdecfa9621f8693586fc48d17cb