Remove needless manual default impl of ButtonBundle (#6970)

# Objective

- Remove a manual impl block for something that can be derived
- Correct a misleading doc comment.

crates/bevy_ui/src/node_bundles.rs

@@ -71,7 +71,7 @@ pub struct ImageBundle {
     pub calculated_size: CalculatedSize,
     /// The background color, which serves as a "fill" for this node
     ///
-    /// When combined with `UiImage`, tints the provided image.
+    /// Combines with `UiImage` to tint the provided image.
     pub background_color: BackgroundColor,
     /// The image of the node
     pub image: UiImage,
@@ -178,7 +178,7 @@ impl Default for TextBundle {
 }
 
 /// A UI node that is a button
-#[derive(Bundle, Clone, Debug)]
+#[derive(Bundle, Clone, Debug, Default)]
 pub struct ButtonBundle {
     /// Describes the size of the node
     pub node: Node,
@@ -213,22 +213,3 @@ pub struct ButtonBundle {
     /// Indicates the depth at which the node should appear in the UI
     pub z_index: ZIndex,
 }
-
-impl Default for ButtonBundle {
-    fn default() -> Self {
-        ButtonBundle {
-            button: Button,
-            interaction: Default::default(),
-            focus_policy: Default::default(),
-            node: Default::default(),
-            style: Default::default(),
-            background_color: Default::default(),
-            image: Default::default(),
-            transform: Default::default(),
-            global_transform: Default::default(),
-            visibility: Default::default(),
-            computed_visibility: Default::default(),
-            z_index: Default::default(),
-        }
-    }
-}