Check missile source type to determine death reason

[StephenCWills]

This resolves an issue where Runes could be used to PK another player and have them drop their equipment as if they had been killed by a trap or monster.

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TARGET_BOTH is only used for a handful of missiles:

• Fire Wall
• Lightning Wall
• Ring of Fire
• Runes
• Phasing

For Fire Wall, Lightning Wall, and Ring of Fire, we were already checking missile.IsTrap() which is equivalent to missile.sourceType() == MissileSource::Trap. Therefore, the behavior after the change should match what we were doing before. Phasing is harmless, so the only real change is that Rune missiles will now also use DeathReason::Player.

This logic also just makes more sense compared to the special case for Fire Wall and Lightning Wall.