Remove `bevy_pbr::generate_view_layouts`

[rambip]

I was surprised to see that generate_view_layouts is not used at all in the repository, it would be nice to remove it.

The more useful function, layout_entries, is private.

The best thing to do IMO is to make layout_entries public and to deprecate generate_view_layouts.

[Utope]

looking at this right now. generate_view_layouts is referenced in this comment. This struct is referenced in a bunch of other places so I can't just remove it. I don't know enough about this system how should i reword it?

bitflags::bitflags! {
    /// A key that uniquely identifies a [`MeshPipelineViewLayout`].
    ///
    /// Used to generate all possible layouts for the mesh pipeline in [`generate_view_layouts`],
    /// so special care must be taken to not add too many flags, as the number of possible layouts
    /// will grow exponentially.
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    #[repr(transparent)]
    pub struct MeshPipelineViewLayoutKey: u32 {
        const MULTISAMPLED                = 1 << 0;
        const DEPTH_PREPASS               = 1 << 1;
        const NORMAL_PREPASS              = 1 << 2;
        const MOTION_VECTOR_PREPASS       = 1 << 3;
        const DEFERRED_PREPASS            = 1 << 4;
        const OIT_ENABLED                 = 1 << 5;
    }
}

[Utope]

realized you said to deprecate not remove it. So I'm not sure if we should change the comment yet. going to create a pr.