Partially reverts commit 5754dbd5.

The code in the specfile adds a MAC address to every <bridge>,
even for <forward mode='bridge'> for which we don't support
changing MAC addresses.

Remove it completely. For new networks, we have been adding
MAC addresses on definition/creation since the commit mentioned above.
For existing networks (pre-0.9.0), the MAC is added by this commit.

https://bugzilla.redhat.com/show_bug.cgi?id=1156367