XRManager: Simplify MSAA setup.

[Mugen87]

Related issue: #30459

Description

Since #30459 XR uses an intermediate framebuffer target which means the XR render target (or the output target) does not require MSAA.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.42 78.33	336.42 78.33	+0 B +0 B
WebGPU	518.01 143.92	518.01 143.92	+0 B +0 B
WebGPU Nodes	517.48 143.81	517.48 143.81	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.41 112.17	465.41 112.17	+0 B +0 B
WebGPU	589.89 160.02	589.89 160.02	+0 B +0 B
WebGPU Nodes	545.25 149.56	545.25 149.56	+0 B +0 B