Default backend

[mbrobbel]

Currently the cli defaults to qx if no backend is specified. We also have this test:

#[test]
fn no_backend() {
    assert!(err!(cli!(FRONTEND)).contains(
            "While interpreting plugin specification: Invalid argument: could not find plugin executable 'dqcsbeqx', needed for plugin specification 'qx'",
    ));
}

which fails when dqcsbeqx is installed.

My suggestion is to remove this default behavior. We can also remove the test, but no default behavior results in less confusing for users, and we're talking about 3 characters.