Remove mod.rs in scene_viewer (#8582)

# Objective

- Cleanup file tree

## Solution

A mysterious mod.rs lies in the scene_viewer directory. It seems
completely useless, everything ignores it and it doesn't affect
anything.

We cruelly remove it, making the world a less whimsical place. A
dystopian drive for pure and complete order compels us to eliminate all
that is useless, for clarity and to prevent the wonder and beauty of
confusion.

examples/tools/scene_viewer/mod.rs

@@ -1,2 +0,0 @@
-pub mod camera_controller_plugin;
-pub mod scene_viewer_plugin;