Remove dup firewall_driver when using bm driver.

When using the nova BM driver (on the undercloud) we have
a duplicated firewall_driver in our nova.conf file.

This commit removes the nova baremetal specific version since
we alread set it around line 60.

Change-Id: I0b06b72a39463dd978d18d8d76771f49dbbecd64
Merge "Remove dup firewall_driver when using bm driver."