element.py: Always clean up the rootdir

We shouldn't need it to persist now that we cache failed build dirs.

This change breaks the test
`tests/integration/shell.py::test_sysroot_workspace_visible`.
I can no longer see a use-case for this test.
AIUI, it tested that the failed build sysroot stored in the builddir has
the workspace's files in, despite the workspace being unmounted.
I believe this behaviour is made redundant by cached buildtrees.

This fixes part of #539