For Ironic CI: VIRT_DRIVER configuration will be write into local.conf twice

[watanabe.isao]

When configuring CI for Ironic using devstack gate job, VIRT_DRIVER configuration will be write into local.conf twice.

The first time is :
in the middle of setup_localrc function [1].
VIRT_DRIVER will be set to DEVSTACK_GATE_VIRT_DRIVER, which should be "ironic".

The second time is:
also in the middle of setup_localrc function (but latter than the first time) [2].
When DEVSTACK_GATE_VIRT_DRIVER is "ironic", "VIRT_DRIVER=ironic" will be right into local.conf again.

I mean we don't need to double the work, even it is no harm to the gate.

[1]https://github.com/openstack-infra/devstack-gate/blob/7026584d219f4dbc5ba1d2475bf3f486083fff6a/devstack-vm-gate.sh#L299
[2]https://github.com/openstack-infra/devstack-gate/blob/7026584d219f4dbc5ba1d2475bf3f486083fff6a/devstack-vm-gate.sh#L341

[watanabe.isao (watanabe.isao)]

Just a one line simple remove. If it is OK. I would like to start the work tomorrow.