Revert "checkasm: vp9dsp: Benchmark the dc-only version of idct_idct separately"

This reverts commit 81d7f0bbca837afda1f7e60d3ae52ab1360ab44b.

Instead of just benchmarking dc separately, test all relevant subparts
(in the next commit).

Signed-off-by: Martin Storsjö <martin@martin.st>
