ovs-port: fix removal of ovsdb entry if the interface goes away

Hope third time is the charm.

The idea here is to remove the OVSDB entry if the device actually went away
violently (like, the it was actually removed from the platform), but keep it if
we're shutting down.

Fixes-test: @ovs_nmstate
Fixes: 966413e7 ('ovs-port: avoid removing the OVSDB entry if we're shutting down')
Fixes: ecc73eb2 ('ovs-port: always remove the OVSDB entry on slave release')

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