meta-phosphor: remove avahi upgrade workaround

A workaround was added in 2019 to handle code updates, which
needed to add an avahi user.  Assume that this has been applied
to all vendor machines by now and can be removed.

Tested: Booted Witherspoon in QEMU and observe the avahi user and group
are still created correctly.

```
root@witherspoon:~# grep avahi /etc/group /etc/passwd
/etc/group:avahi:x:996:
/etc/passwd:avahi:x:997:996::/run/avahi-daemon:/bin/false
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I704dd7f09803df32690ba37882e94bb0664597e5