aom: remove jpeg-xl & libvmaf dependencies

[gianni-rosato]

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Removes the jpeg-xl and libvmaf dependencies from libaom.

• jpeg-xl is currently unused; it was used for Butteraugli support, which is no longer included in the libjxl repo or meaningfully utilized within libaom.
• libvmaf provides support for libaom's previously experimental VMAF tune for video. This tune is functionally dead at this point, and is no longer under active development. The flagship tunes for video remain tune=ssim and tune=psnr.