Removing unused property in BatchedMesh

[andredsm]

This PR only removes _multiDrawInstances definition from BatchedMesh class, since PR #28404 removed all code that has been using it but kept the property definition.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	690.29 171.01	690.26 171	-30 B -6 B
WebGPU	815.35 219.78	815.32 219.77	-30 B -7 B
WebGPU Nodes	814.86 219.65	814.83 219.64	-30 B -6 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	463.31 111.89	463.31 111.89	+0 B +0 B
WebGPU	538.09 145.28	538.09 145.28	+0 B +0 B
WebGPU Nodes	494.2 135.15	494.2 135.15	+0 B +0 B