Allow beams to do energy leeching

[Baezon]

I considered having shockwaves do this too, but having "esuck" missiles suddenly start draining double the amount is probably not a good idea... so a problem for later.

Additionally, this removes the Beams_use_damage_factors check from shiphit_get_damage_weapon, the purpose of that function is to return the wip_index of the provided object, not to withhold it based on a specific usage. All locations in the code where this would be relevant already check this flag, or have been made to do so by this PR (indeed some unrelated feature, like "vampiric" have been inadvertently trapped behind this flag as a result).