Some hostile mob code AI cleanup

[FluffyGhoster]

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Fixed some situations where a null was being set as target. Repathed the dreaded target_mob variable to last_found_target, and from mob to atom, as it was not being used only for mobs. Added procs to set and unset the last_found_target and setup the signals relative to clear it, and validations. Thanks to the above, removed a viewscan that was needed to delete references when mobs die from their attacker. General QoL of code, DMDocs.