Removed AudioManager

Initially I thought there were going to be a lot more reused sounds, but
it turns out most sounds are used by one entity, and the ones that arnt
are part of the level, which facilitates it's own way of loading in
sound files only once. This system is more hassle than it's worth, so i
removed it.

This does however mean that if there is a repeated sound, you have to
manage the memory yourself, but it's pretty trivial.