Implement attribute unsetting logic

[nosoop]

The docs tell of a keep_static_attrs mode that removes gameplay-affecting attributes.

That's on the to-do list.

[nosoop]

This one's tricky, since the Flamethrower (of course) does have static attributes that do have value types such as flame_reflect_on_collision, so we can't just go through and overlay everything with 0.0 (for additive) or 1.0 (for multiplicative).

One way to get around this could be to do a second pass and apply the attributes from the class's default item for that slot. Might just have to try that.