[TRACKER] Clearing technical debt of RF/DT code

[teju85]

With the old backend finally removed from cuML, we now only have the code from new backend being used. However, we still have quite a few tech-debt from the old code. This task-tracking issue lists all those to be paid-off over the next few weeks/releases:

• [ ] Just like how #3854 unifies classification/regression paths into one, we could do something similar for the fit/predict methods in decisiontree.cu and by extension all the "impl" files in the same folder.
• [ ] IMO, there's no need for creating multiple internal wrapper classes (DecisionTreeRegressor, DecisionTreeClassifier and DecisionTreeBase) and their corresponding declarations. Instead, just have fit/predict functions in decisiontree.cu directly call the internal kernels/functions without having to go through these classes. Folks, let me know if in case I'm missing the point of having these internal classes.
• [ ] Same set of above ideas could also apply to our randomforest folder.
• [ ] #3089 is looking at cython side of tech-debt's (Thanks to Venkat)
• [ ] Consolidation of google tests. There are tests floating around in a few files with duplicated fixtures. Fil tests are a good example of how it should be done. (Thanks to Rory)

More items to be added over the course of time.

[teju85]

Tagging @vinaydes, @venkywonka and @RAMitchell also to hear their thoughts on this. Let me know in case I'm missing any other tech-debt items from the above list.

[venkywonka]

linking issue #3089 as it is related

[RAMitchell]

Consolidation of google tests. There are tests floating around in a few files with duplicated fixtures. Fil tests are a good example of how it should be done.

[teju85]

Thank you Rory and Venkat for the items. I have updated these in the description above.

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