Finish removal of Position

The component has been completely removed, and all call sites are fixed
to use the Transform directly.

src/physics.rs

@@ -5,9 +5,6 @@ use crate::WorldSize;
 
 use bevy::prelude::*;
 
-#[derive(Component)]
-pub(crate) struct Position(pub(crate) bevy::math::Vec2);
-
 #[derive(Component)]
 pub(crate) struct Velocity(pub(crate) bevy::math::Vec2);