Remove face_toward.rs (bevyengine#4277)

# Objective

- Part of the splitting process of bevyengine#3503.

## Solution

- Remove the `face_toward.rs` file containing the `FaceToward` trait.

## Reasons

- It is unused inside of `bevy`.
- The method `Mat4::face_toward` of the trait is identical to `Mat4::look_at_rh` (see https://docs.rs/glam/latest/glam/f32/struct.Mat4.html#method.look_at_rh).
- Discussion in bevyengine#3503.

## Changelog

### Removed

- The `FaceToward` trait got removed.

## Migration Guide

-  The `FaceToward` trait got removed. To migrate you just have to change every occurrence of `Mat4::face_toward` to `Mat4::look_at_rh`.

crates/bevy_math/src/lib.rs

@@ -1,14 +1,12 @@
-mod face_toward;
 mod geometry;
 
-pub use face_toward::*;
 pub use geometry::*;
 pub use glam::*;
 
 pub mod prelude {
     #[doc(hidden)]
     pub use crate::{
-        BVec2, BVec3, BVec4, EulerRot, FaceToward, IVec2, IVec3, IVec4, Mat3, Mat4, Quat, Rect,
-        Size, UVec2, UVec3, UVec4, Vec2, Vec3, Vec4,
+        BVec2, BVec3, BVec4, EulerRot, IVec2, IVec3, IVec4, Mat3, Mat4, Quat, Rect, Size, UVec2,
+        UVec3, UVec4, Vec2, Vec3, Vec4,
     };
 }