Remove subclasses from composer

[bmosaicml]

What does this PR do?

This PR removes/deprecates the ICL(Dataset|Metric) subclasses and migrates the relevant tests. This PR is not strictly necessary but would help prevent confusion about where ICL code actually lives. It's related to this foundry PR: mosaicml/llm-foundry#936

What issue(s) does this change relate to?

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