tests/amdgpu/vcn: clean abundant codes

Remove useless codes.

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
6 jobs for ref/for/master in 40 seconds (queued for 1 second)
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debian

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meson-aarch64

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