fix shader_instancing (#7305)

# Objective

- The changes to the MeshPipeline done for the prepass broke the shader_instancing example. The issue is that the view_layout changes based on if MSAA is enabled or not, but the example hardcoded the view_layout.

## Solution

- Don't overwrite the bind_group_layout of the descriptor since the MeshPipeline already takes care of this in the specialize function.

Closes https://github.com/bevyengine/bevy/issues/7285

examples/shader/shader_instancing.rs

@@ -205,11 +205,6 @@ impl SpecializedMeshPipeline for CustomPipeline {
             ],
         });
         descriptor.fragment.as_mut().unwrap().shader = self.shader.clone();
-        descriptor.layout = Some(vec![
-            self.mesh_pipeline.view_layout.clone(),
-            self.mesh_pipeline.mesh_layout.clone(),
-        ]);
-
         Ok(descriptor)
     }
 }