Remove duplicate functionality of Enemy and Player collision events

[tigleym]

This issue looks at removing the duplicated logic done in https://github.com/amethyst/space_shooter_rs/blob/master/src/events/events.rs#L27-L67.

Ideally, we should have one general collision event to simplify things. Some approaches we can take here is to either pass enums values through the collision event channel, or, attach component tags to entities to identify different collision types.

This should also help us reduce amount of "A-to-B" / "B-to-A" collision systems that are present in enemy_collisions.rs and spaceship_collisions.rs.