Remove parallel iterators in attribute processors (#4999)

At least on my machine, the parallel iterators that were recently added
to the attribute processing methods result in at least an order of
magnitude performance loss.

With `par_iter`:

![image](https://github.com/rerun-io/rerun/assets/2910679/f2bbcace-2965-4ecf-b35d-ff565c6b0d98)


Without `par_iter`:

![image](https://github.com/rerun-io/rerun/assets/2910679/2eda2b43-4ab9-418e-a596-3d36ebb03027)

(Screenshots taken from #5000)

crates/re_space_view_spatial/src/visualizers/mod.rs

@@ -132,7 +132,6 @@ pub fn process_color_slice<'a>(
 ) -> Vec<egui::Color32> {
     // This can be rather slow for colors with transparency, since we need to pre-multiply the alpha.
     re_tracing::profile_function!();
-    use rayon::prelude::*;
 
     let default_color = DefaultColor::EntityPath(ent_path);
 
@@ -146,7 +145,7 @@ pub fn process_color_slice<'a>(
         (None, ResolvedAnnotationInfos::Many(annotation_infos)) => {
             re_tracing::profile_scope!("no-colors, many annotations");
             annotation_infos
-                .par_iter()
+                .iter()
                 .map(|annotation_info| annotation_info.color(None, default_color))
                 .collect()
         }
@@ -155,16 +154,16 @@ pub fn process_color_slice<'a>(
             re_tracing::profile_scope!("many-colors, same annotation");
             debug_assert_eq!(colors.len(), *count);
             colors
-                .par_iter()
+                .iter()
                 .map(|color| annotation_info.color(color.map(|c| c.to_array()), default_color))
                 .collect()
         }
 
         (Some(colors), ResolvedAnnotationInfos::Many(annotation_infos)) => {
             re_tracing::profile_scope!("many-colors, many annotations");
             colors
-                .par_iter()
-                .zip(annotation_infos.par_iter())
+                .iter()
+                .zip(annotation_infos.iter())
                 .map(move |(color, annotation_info)| {
                     annotation_info.color(color.map(|c| c.to_array()), default_color)
                 })
@@ -209,16 +208,14 @@ pub fn process_radius_slice(
     ent_path: &EntityPath,
 ) -> Vec<re_renderer::Size> {
     re_tracing::profile_function!();
-    use rayon::prelude::*;
-
     let ent_path = ent_path.clone();
 
     match radii {
         None => {
             vec![re_renderer::Size::AUTO; default_len]
         }
         Some(radii) => radii
-            .par_iter()
+            .iter()
             .map(|radius| process_radius(&ent_path, radius))
             .collect(),
     }