Consider removing `RetainedViewEntity`

[beicause]

RetainedViewEntity is used as a stable identifier across frames

bevy/crates/bevy_render/src/view/mod.rs

Lines 268 to 278 in 25368b7

	/// An identifier for a view that is stable across frames.
	///
	/// We can't use [`Entity`] for this because render world entities aren't
	/// stable, and we can't use just [`MainEntity`] because some main world views
	/// extract to multiple render world views. For example, a directional light
	/// extracts to one render world view per cascade, and a point light extracts to
	/// one render world view per cubemap face. So we pair the main entity with an
	/// *auxiliary entity* and a *subview index*, which *together* uniquely identify
	/// a view in the render world in a way that's stable from frame to frame.
	#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
	pub struct RetainedViewEntity {

Since #18722 is fixed, the render entity across frames should be stable now. We can try replacing RetainedViewEntity with Entity and use faster EntityHasher.

Also ViewKeyCache may be converted from a Hashmap to a Component.