call monster::die instead of game::kill_mon in throwing code to record the killer

Fixes #5800

Soyweiser is right, it did not depend on the distance to dead zombie.

The throwing code had not been updated to the new damage system, which uses functions like `Creature::projectile_attack` and tracks the attacker. Instead it still deals the damage directly and does not track who hurt/killed the monster.

This PR changes the call from `game::kill_mon` to `monster::die`. `monster::die` calls `game::kill_mon` anyway *and* it records the killer was you.