Drop armhf for ovn package

[Frode Nordahl]

Hello,

Debian has decided to mark the ovn package as 64-bit only [0] on the
back of [1].

This is currently preventing proposed migration [2][3] in Mantic and
I'd like to propose we drop armhf for this package starting with
Mantic.

0: https://salsa.debian.org/openstack-team/third-party/ovn/-/commit/58d832f66ac22fad688e514abbe030331e50cfe2
1: https://bugs.launchpad.net/neutron/+bug/2025985
2: https://launchpad.net/ubuntu/+source/ovn/23.03.0-2
3: https://ubuntu-archive-team.ubuntu.com/proposed-migration/mantic/update_excuses.html#ovn

-- 
Frode Nordahl