Use default optimizerSteps when using viaIR

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Currently optimizerSteps: "u" is a recommended way to configure viaIR compilation pipeline. However it might not be necessary anymore with recent versions of solidity. The suggestion of this issue is to switch to using compiler-default optimizerSteps wherever possible and maybe even remove the recommendation of using optimizerSteps: "u" from the docs.

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Edit from @fvictorio: before making this recommendation in the docs, the stack traces tests need to be run without those optimizer steps. We know that our tests fail wihout them for previous versions, but if they work fine in the latest N versions (where N is TBD), then we can change the recommendation.

This means that we need to move the optimizer steps to the per-version test config, use "u" for the ones that need it, and remove it for the ones that don't.