Remove face_toward.rs (#4277)

# Objective

- Part of the splitting process of #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 #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/face_toward.rs

@@ -1,41 +0,0 @@
-use crate::{Mat4, Vec3};
-
-/// Generates a translation / rotation matrix that faces a given target
-pub trait FaceToward {
-    /// Generates a translation / rotation matrix that faces a given target
-    fn face_toward(eye: Vec3, center: Vec3, up: Vec3) -> Self;
-}
-
-impl FaceToward for Mat4 {
-    fn face_toward(eye: Vec3, center: Vec3, up: Vec3) -> Self {
-        let forward = (eye - center).normalize();
-        let right = up.cross(forward).normalize();
-        let up = forward.cross(right);
-        Mat4::from_cols(
-            right.extend(0.0),
-            up.extend(0.0),
-            forward.extend(0.0),
-            eye.extend(1.0),
-        )
-    }
-}
-
-#[cfg(test)]
-mod test {
-    #[test]
-    fn face_toward_mat4() {
-        use crate::{FaceToward, Mat4, Vec3, Vec4};
-
-        // Completely arbitrary arguments
-        let matrix = Mat4::face_toward(
-            Vec3::new(50.0, 60.0, 0.0),
-            Vec3::new(0.0, 0.0, 0.0),
-            Vec3::new(0.0, 1.0, 0.0),
-        );
-
-        assert_eq!(matrix.x_axis, Vec4::new(0.0, 0.0, -1.0, -0.0));
-        assert_eq!(matrix.y_axis, Vec4::new(-0.7682213, 0.6401844, 0.0, 0.0));
-        assert_eq!(matrix.z_axis, Vec4::new(0.6401844, 0.7682213, 0.0, 0.0));
-        assert_eq!(matrix.w_axis, Vec4::new(50.0, 60.0, 0.0, 1.0));
-    }
-}

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,
+        BVec2, BVec3, BVec4, EulerRot, IVec2, IVec3, IVec4, Mat3, Mat4, Quat, Rect, Size, UVec2,
-        Size, UVec2, UVec3, UVec4, Vec2, Vec3, Vec4,
+        UVec3, UVec4, Vec2, Vec3, Vec4,
     };
 }