Fixed explosion slowdown issue

After a certain amount of time, explosions would gather up enough that
is caused slowdown; to reproduce, revert this change, spawn one tank,
then wait about a minute and try to move the view left or right.
Applying this change removes the issue; spawning a tank and letting it
do its thang for a minute does not cause slowdown. The condition for deletion is that the size of one of the particles in the explosion be less than or equal to zero, in which case you can't see it anymore.