Cleanup cuml.multiclass

[jcrist]

This:

• Cleans up our multiclass implementations slightly. In the future we'll want to do redo these implementations so we're not just wrappers around sklearn (requiring host transfers etc...), but for now at least we're following conventions in [TRACKER] Cleanup python estimator implementations #7317.
• Deprecates MulticlassClassifier in favor of OneVsOneClassifier/OneVsRestClassifier. This is to reduce our estimator maintenance a bit - sklearn only provides the latter 2, no need for us to add another estimator class.

Part of #7317.

[aamijar]

lgtm!

[jcrist]

/merge