ovs-port: avoid removing the OVSDB entry if we're shutting down

Since commit ecc73eb2 ('ovs-port: always remove the OVSDB entry on
slave release'), ovs port were removing the ovsdb entry upon being
un-enslaved, no matter what the reason for un-enslavement was. The idea
was to remove the stale ovsdb entry upon forcible device removal.

This cleanup is specific to OpenVSwitch, since for other device types,
the device master is the property of the slave and thus goes away along
with the device.

Turns out we're now removing the ovsdb entry even when the device
actually doesn't go away, but we're pretending it does because the
daemon is shutting down.

To add insult to injury, we generally end up removing one entry,
because the other ovsdb calls end up in a queue and don't get serviced
before the daemon shuts down. The result is a mess. (This patch
doesn't solve that -- if someone terminates the daemon with in-flight
ovsdb calls they're still out of luck).

Let's do the cleanup now only if the device was actually physically
removed.

https://bugzilla.redhat.com/show_bug.cgi?id=2055665

Fixes-test: @NM_reboot_openvswitch_vlan_configuration
Fixes: ecc73eb2 ('ovs-port: always remove the OVSDB entry on slave release')
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