ci: Drop the prepare.sh script

ci: Drop the prepare.sh script

The purpose of this script was to prepare a customized environment in
the container, but was actually never used and it required the usage of
sudo to switch the environment from root's context to a regular user's
one.
The thing is that once someone needs a custom script they would very
likely to debug something and would also benefit from root privileges
in general, so the usage of 'sudo' in such case was a bit cumbersome.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>