Fix removal of mission items

Fixes https://github.com/CleverRaven/Cataclysm-DDA/pull/9393#discussion_r18550107

The problem was/is that `charges_of` (called by `has_charges`) considers item that are not counted by charges as 1 charges (e.g. `charges_of("2x4")` returns 1 if the player has a single 2x4 in their inventory). This leads to `game::wrap_up_mission` thinking the item is counted by charges and removes the items by charges.

Fixed this by checking the count-by-charges on a temporary item.

Also fixed the misleading comment as pointed out there: https://github.com/CleverRaven/Cataclysm-DDA/pull/9393#discussion_r18564596