[SPARK-11719] [ML] Remove duplicate DecisionTreeExample under examples/ml

[yanboliang]

We have DecisionTreeClassificationExample and DecisionTreeRegressionExample for Scala/Java/Python under examples/ml, so removed duplicate DecisionTreeExample.

[SparkQA]

Test build #45815 has finished for PR 9682 at commit e0c0c3f.

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• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #45820 has finished for PR 9682 at commit e0c0c3f.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

@yanboliang There are two types of example code under examples/:

1. Example applications (the ones with command-line parsing). Those are served as template code for users to build their own standalone applications. We should keep those for important algorithms.
2. Tutorial code. Those are for the user guide, usually without options to play with. We should keep at most one copy per algorithm.

[yanboliang]

I think we should keep the DecisionTreeExample, so closing this PR. Thanks for your comments. @mengxr