Drops a weapon.
Function which verifies existence of _item and _unit, returns false in case of trouble, or when able to remove _item from _unit true in case of success
|_unit||the unit that should drop a weapon <OBJECT>|
|_item||class name of the weapon to drop STRING|
true if successful, false otherwise <BOOLEAN>
_result = [player, primaryWeapon player] call CBA_fnc_dropWeapon