Fix #214 (Crushstacean + Fireball Player Removal)

Fireballs and Shells currently set the tracer of objects they kill to themselves. This is in order to play sfx_mario2 when A_Scream is called, but it can often lead to unintended side effects, such as #214 (closed).

This merge request fixes the problem by not setting the tracer and just playing sfx_mario2 outright. A_Scream will not be executed if it detects sfx_mario2 is being played by the object that called it, thus keeping the original behavior mostly intact, except in a few edge cases.