[2022.3][AppSW] Fix incorrect motionVector pass check

[TrevorDaschOculus]

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Purpose of this PR

Instead of checking whether the motionVectorRenderTarget is equal to -1, simply copy the 'hasMotionVectorPass' from the engine's XRRenderPass.

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Testing status

Confirmed that MotionVector pass executes when expected and is skipped when not expected

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