Changes

testenv: remove dead code for [testsuite] prepare=

Remove some WIP code that I intended to use for enabling the mongodb
repository before installing binary packages, to be able to dynamically
install mongodb from there. I solved it differently by just always
having mongodb in the podman image.

This was a dead code path because configs with prepare= in [testsuite]
are currently not valid, and therefore testenv refuses to use these
configs (see keys_valid_testsuite in testenv_cfg.py's verify()).

I have a different use case for running prepare= before running the
testsuite, to replace a module parameter in the testsuite's config. This
will be done in the next patch.

Change-Id: I9bed48612fad690f86017c298372dc6cea22b94b