Fix references to icons/goods/missile_*.png

These references come from the equipment market screen. Specifically, when the ship's equipment table is updated, each row gets an icon. If the equipment type (in libs/Equipment.lua) does not specify an icon_name value, then the icon is left empty (this is what happens for most equipment types). If the equipment type does specify an icon_name (which the missiles do, because missile icons are needed elsewhere in the UI), then the code attempts to load `"icons/goods/" .. icon_name .. ".png"`. This outputs a warning message when trying to load the missile icons, because those icons are in data/icons, not data/icons/goods.

But... having done all that work to construct a UI Image widget and (try to) load an icon file, that widget is never actually used, and will just be destroyed again by the Lua garbage collector (at least, it *should* be destroyed again -- I haven't tried to check this).