WebGPURenderer: remove obsolete code

[aardgoose]

RenderList.getLightsNode() has no callers and calls a LightsNode method that has been removed.
Remove obsolete method.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	689.82 170.9	689.82 170.9	+0 B +0 B
WebGPU	811.56 218.6	811.49 218.59	-68 B -14 B
WebGPU Nodes	811.07 218.47	811 218.46	-68 B -14 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.84 111.78	462.84 111.78	+0 B +0 B
WebGPU	535.78 144.59	535.71 144.58	-68 B -17 B
WebGPU Nodes	491.89 134.34	491.83 134.32	-68 B -18 B