Remove margins.rs (bevyengine#4284)

# Objective

- Closes bevyengine#335.
- Part of the splitting process of bevyengine#3503.

## Solution

- Remove the `margins.rs` file containing the `Margins` type.

## Reasons

- It is unused inside of `bevy`.
- The `Rect`/`UiRect` is identical to the `Margins` type and is also used for margins inside of `bevy` (rename of `Rect` happens in bevyengine#4276)
- Discussion in bevyengine#3503.

## Changelog

### Removed

- The `Margins` type got removed.

## Migration Guide

- The `Margins` type got removed. To migrate you just have to change every occurrence of `Margins` to `UiRect`.

crates/bevy_ui/src/lib.rs

@@ -4,7 +4,6 @@
 //! This UI is laid out with the Flexbox paradigm (see <https://cssreference.io/flexbox/> ) except the vertical axis is inverted
 mod flex;
 mod focus;
-mod margins;
 mod render;
 mod ui_node;
 
@@ -15,14 +14,13 @@ pub mod widget;
 use bevy_render::camera::CameraTypePlugin;
 pub use flex::*;
 pub use focus::*;
-pub use margins::*;
 pub use render::*;
 pub use ui_node::*;
 
 #[doc(hidden)]
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{entity::*, ui_node::*, widget::Button, Interaction, Margins};
+    pub use crate::{entity::*, ui_node::*, widget::Button, Interaction};
 }
 
 use bevy_app::prelude::*;