Revert "Use the new net.ifnames kernel command line flag to disable persistent interface names"

As @edolstra pointed out, this behavior is not equivalent to what we had
before as the kernel command line parameter won't take effect until the
next boot. Probably it's not likely that someone will make this change
and then add a network card before rebooting, but might as well support
that since we can.

This reverts commit f7563698

.

Signed-off-by: Shea Levy <shea@shealevy.com>