build-setup.sh: remove new default ubuntu user

If the user running build-setup.sh happens to have a UID of 1000 (which
is the case in a lot of dedicated CI machines) then it will no longer be
able to build OpenBMC using the latest Ubuntu containers due to a new
default "ubuntu" user being added to the images.

It will get the following error when trying to start the container:
```
useradd: UID 1000 is not unique
```

This new "feature" is documented up here:
  https://bugs.launchpad.net/cloud-images/+bug/2005129

This change utilizes the solution put forth by the bug owner of just
deleting the new default user. It does result in the following harmless
warning:
```
userdel: ubuntu mail spool (/var/mail/ubuntu) not found
```

A different solution was chosen in 67cc06169 but it's not working in
this sh script so stick to the simple solution here.

Tested:
- Confirmed build-setup.sh runs again with users that have a UID of 1000

Change-Id: I3f8c691fad2778a17e23c2d4e45eb86af7a78556
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
1 file changed