Resize Ubuntu 18.04 builders to default of 20GB

The new Ubuntu images that we recently changed over to require at
least 20GB of HDD, rather than the previous setting of 10GB. By
removing the value from these files, we will use the config default
of 20GB.

Change-Id: I7259d1321f315d47b2fa8d8fce3c8b83543c8f3b
Signed-off-by: Eric Ball <eball@linuxfoundation.org>