Clean up specialized_mesh_pipeline

[IceSentry]

Objective

• The example doesn't work on webgl2
• It tries to reimplement logic for batching instead of using existing abstractions which breaks on webgl2
• It only uses batching

Solution

• Use existing abstractions and remove code related to batching
  • This fixes the webgl2 issue
  • It also makes it use multi draw indirect instead of just batching

Testing

• Tested the example with the bevy cli for webgl and wegpu and also ran the example locally