Fix being able to ctrl-k external factories

[lL1l1]

Issue

You could ctrl-k external factories and they would be gone forever.

Description of the proposed changes

• Sets CanBeKilled to false so that the factories cannot die.
• Since they cannot be killed anymore, Destroy must be called. External factories have no visuals or gameplay so none of that code should apply. @relent0r said that AI uses .ExternalFactory on the parent unit to interact with external factories, so the OnKilled callback no longer firing should also not be relevant.

Testing done on the proposed changes

• Ordering an external factory to kill itself does not do anything.
• External factories are destroyed when the parent dies. Without the relevant change, they were draining resources while the parent unit was dead but not destroyed (for example when sinking).

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in the changelog for the next game version