ProjectorLightNode: Remove obsolete toVar().

[Mugen87]

Related issue: #31459

Description

Thanks to #31459, the code in ProjectorLightNode.getSpotAttenuation() can be simplified.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.92 78.83	337.92 78.83	+0 B +0 B
WebGPU	560.22 155.11	560.22 155.1	-8 B -5 B
WebGPU Nodes	559.15 154.89	559.14 154.89	-8 B -4 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	469.19 113.49	469.19 113.49	+0 B +0 B
WebGPU	635.39 172.03	635.38 172.03	-8 B -1 B
WebGPU Nodes	590.04 161.27	590.03 161.27	-8 B -2 B