z_layer_2d is both confusing and buggy, should be removed

[djeedai]

Crate versions

main

Describe the bug

z_layer_2d for 2D effects is used in place of the transform.z, which 1) makes things confusing for users, and 2) seems buggy with parenting. There's really no reason for having it. Bevy's Mesh2d and Sprite pass the transform.z as the Transparent2d.sort_key value, and Hanabi should do the same.

Expected behavior

Particle effect with larger Z position renders on top in 2D. That's not currently always the case.

To Reproduce

See 2d example; disable the animation system update_plane() and change z_layer_2d and transform.z of the particle effect: the former moves the particles in front of and behind the quad, the latter does nothing.