python: mkvenv: remove ensure command

This was used to bootstrap the venv with a TOML parser, after which
ensuregroup is used.  Now that we expect it to be present as a system
package (either tomli or, for Python 3.11, tomllib), it is not needed
anymore.

Note that this means that, when implemented, the hypothetical "isolated"
mode that does not use any system packages will only work with Python
3.11+.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>