Fix #4648: Update AT Classes for MekMortar and IS BA Tube Arty

[Sleet01]

It turns out that Mek Mortars (and, coincidentally, IS BA Tube Artillery) are filtered out of the MekHQ "Spend XP -> SPA" context menu because they have the AT Class "CLASS_NONE". Because MekHQ uses a different code path to validate weapon validity for SPAs, these weapons are filtered out in MekHQ but work fine in MegaMek.

This patch by itself only fixes the issue for IS BA Tube Artillery; the matching MekHQ PR here must also be applied to fix the issue for Mek Mortars as well.

We should probably look at:

1. removing all AT Class filtering, as it is apparently quite old and mostly unused,
2. importing the MegaMek SPA validity test, which is much more robust, into MekHQ.

but I have some other stuff to work on right now.

Tested with MegaMek and MekHQ; ran all tests on both packages.

Close #4648

[NickAragua]

Any idea where these values are used and what effect changing it will have?

Also, why do we have two "AMS" (one on line 253, one on line 255)?

[Sleet01]

Looking at the code and commits, it looks like AT Classes were an attempt at separating AeroTech2 or Naval weapons from other weapons without expanding stuff like WeaponType (due to bitmap size restrictions).

These AT Classes are apparently used for MML printing and Weapon Bay storing / loading, as well as some logic that determines what kind of Bay will be created by adding different WeaponTypes to a Bay. It's mostly very old Aerospace / Dropship-related code. Adding a new entry shouldn't cause any issues; note that CLASS_GAUSS and CLASS_THUNDERBOLT have already been added without classNames entries.

No idea why there are two AMS entries, it looks like a typo - 6 is PLASMA, not AMS. I can update that...