Fix Miasma shield penetration

[clyfordv]

Issue:
With the airburst on the Miasma, if the child projectile is spawned around the same tick as it would hit a shield, that child projectile would pass through the shield. This was not random, and consistent enough that it's likely exploitable*.

*Edit: forgot that I had the Miasma on perfect accuracy--not exploitable, just annoyingly random.

Solution:
Remove the secondary projectile that spawns with the airburst, and leave all the effects and damage dealing up to the main one.

Other attempted solutions:
I tried moving all damage dealing on the primary projectile while keeping the effects on the secondary (the secondary would still penetrate in some cases, but wouldn't do damage, and though repeatable the odds of it happening accidentally are low so it'd be a minor visual irregularity). The issue with this is spawning a projectile on a projectile, and not destroying the original projectile immediately, causes the strategic icon to stutter, which looks bad.

Afterthoughts:
Ballistic behavior is now in line with other artillery (no airburst). Damage over time remains. Miasma cloud effect is still there, but the impact looks a little uninspired without the secondary projectile emitter trail, and it could use a more impactful (...) visual effect when it hits something.