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>
diff --git a/build-setup.sh b/build-setup.sh
index 1164f15..71b8090 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -263,6 +263,11 @@
   ENV LANGUAGE en_US:en
   ENV LC_ALL en_US.UTF-8
 
+  # Latest Ubuntu added a default user (ubuntu), which takes 1000 UID.
+  # If the user calling this build script happens to also have a UID of 1000
+  # then the container no longer will work. Delete the new ubuntu user
+  # so there is no conflict
+  RUN userdel -r ubuntu
   RUN grep -q ${GROUPS[0]} /etc/group || groupadd -g ${GROUPS[0]} ${USER}
   RUN grep -q ${UID} /etc/passwd || useradd -d ${HOME} -m -u ${UID} -g ${GROUPS[0]} ${USER}