ironic: default to ipxe booting

Enable the Ironic ipxe boot interface by default, following a similar
change in Ironic [1].

Drop the kolla_enable_ironic_ipxe flag, following a similar change in
Kolla Ansible [2]. Both PXE and iPXE are now enabled by default. Users
may revert to using PXE for ironic inspector's dnsmasq, by setting
ironic_dnsmasq_serve_ipxe to false in etc/kayobe/kolla/globals.yml.

[1] https://review.opendev.org/c/openstack/ironic/+/816824
[2] https://review.opendev.org/c/openstack/kolla-ansible/+/834512/

Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/832159
Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/834511
Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/837069

Change-Id: Ifb80bd15a20c9cfb8fbc6e0f6ac23baae631a18e