[DEP] Remove old benchmarking classes

[MatthewMiddlehurst]

This PR removes the BaseBenchmark class. I believe this was primarily for forecasting, but I have not seen it used. I do not think we need a class based structure like this in benchmarking. With this removed there are no usages of kotsu so this has been taken out of all_extras also.

get_qalpha only has one usage, so I have moved it to performance_metrics and made it private.

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