fix: treat 404 on server delete as success

[nilsherzig]

same as in ServerStateRefreshFunc

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Description

What code changed, and why?

Handling of 404 responses when trying to delete an instance the second time around. Simpler Version of #127, might be easier to merge? Similar logic is already used in the project at: https://github.com/hashicorp/packer-plugin-openstack/blob/a7e79ff321ff2b425ea8851b43682f9f2bdbcc63/builder/openstack/server.go#L45

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