CA-110947: Revert "Remove a modprobe warning in new systems."

This breaks switching to the bridge networking backend - this change
means /etc/modprobe.d/blacklist-bridge is not deleted when switching
to bridge, so after the host reboots it can't load the ethernet
bridging kernel module and networking is totally broken.

This reverts commit 37eee6eff0ce3fb4a0185e6f4aa03094c0668844.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1472 from johnelse/ca-110947

Revert "Remove a modprobe warning in new systems."