Get rid of unnecessary mutable access in ui picking backend (#15630)

## Solution

Yeet

## Testing

Tested the `simple_picking` example

crates/bevy_ui/src/picking_backend.rs

@@ -67,7 +67,7 @@ pub fn ui_picking(
  primary_window: Query<Entity, With<PrimaryWindow>>,
  ui_scale: Res<UiScale>,
  ui_stack: Res<UiStack>,
- mut node_query: Query<NodeQuery>,
+ node_query: Query<NodeQuery>,
  mut output: EventWriter<PointerHits>,
 ) {
  // For each camera, the pointer and its position
@@ -119,7 +119,7 @@ pub fn ui_picking(
  // reverse the iterator to traverse the tree from closest nodes to furthest
  .rev()
  {
- let Ok(node) = node_query.get_mut(*node_entity) else {
+ let Ok(node) = node_query.get(*node_entity) else {
  continue;
  };
 
@@ -183,11 +183,10 @@ pub fn ui_picking(
  for ((camera, pointer), hovered_nodes) in hit_nodes.iter() {
  // As soon as a node with a `Block` focus policy is detected, the iteration will stop on it
  // because it "captures" the interaction.
- let mut iter = node_query.iter_many_mut(hovered_nodes.iter());
  let mut picks = Vec::new();
  let mut depth = 0.0;
 
- while let Some(node) = iter.fetch_next() {
+ for node in node_query.iter_many(hovered_nodes) {
  let Some(camera_entity) = node
  .target_camera
  .map(TargetCamera::entity)