Clean up TD weapon ValidTargets

- Missiles can now force-fire on water like other weapons
- Superweapons can now target empty water
- made Chemspray null InvalidTargets to avoid yaml-merge issues
- Improved APCGun effect warhead perf by ignoring actors
- removed stale Tiberium weapon mission overrides

mods/cnc/weapons/smallcaliber.yaml

@@ -148,6 +148,7 @@ APCGun:
 	Warhead@2Eff: CreateEffect
 		Explosions: small_frag
 		ValidTargets: Ground, Water, Air
+		ImpactActors: false
 
 APCGun.AA:
 	Inherits: APCGun