[SPARK-13814] [PYSPARK] Delete unnecessary imports in python examples files

JIRA:  https://issues.apache.org/jira/browse/SPARK-13814

## What changes were proposed in this pull request?

delete unnecessary imports in python examples files

## How was this patch tested?

manual tests

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #11651 from zhengruifeng/del_import_pe.

examples/src/main/python/ml/als_example.py

@@ -21,8 +21,6 @@ from pyspark import SparkContext
 from pyspark.sql import SQLContext
 
 # $example on$
-import math
-
 from pyspark.ml.evaluation import RegressionEvaluator
 from pyspark.ml.recommendation import ALS
 from pyspark.sql import Row

examples/src/main/python/ml/decision_tree_classification_example.py

@@ -20,8 +20,6 @@ Decision Tree Classification Example.
 """
 from __future__ import print_function
 
-import sys
-
 # $example on$
 from pyspark import SparkContext, SQLContext
 from pyspark.ml import Pipeline

examples/src/main/python/ml/decision_tree_regression_example.py

@@ -20,8 +20,6 @@ Decision Tree Regression Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext, SQLContext
 # $example on$
 from pyspark.ml import Pipeline

examples/src/main/python/ml/gradient_boosted_tree_classifier_example.py

@@ -20,8 +20,6 @@ Gradient Boosted Tree Classifier Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext, SQLContext
 # $example on$
 from pyspark.ml import Pipeline

examples/src/main/python/ml/gradient_boosted_tree_regressor_example.py

@@ -20,8 +20,6 @@ Gradient Boosted Tree Regressor Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext, SQLContext
 # $example on$
 from pyspark.ml import Pipeline

examples/src/main/python/ml/kmeans_example.py

@@ -18,7 +18,6 @@
 from __future__ import print_function
 
 import sys
-import re
 
 import numpy as np
 from pyspark import SparkContext

examples/src/main/python/ml/random_forest_classifier_example.py

@@ -20,8 +20,6 @@ Random Forest Classifier Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext, SQLContext
 # $example on$
 from pyspark.ml import Pipeline

examples/src/main/python/ml/random_forest_regressor_example.py

@@ -20,8 +20,6 @@ Random Forest Regressor Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext, SQLContext
 # $example on$
 from pyspark.ml import Pipeline

examples/src/main/python/mllib/binary_classification_metrics_example.py

@@ -18,7 +18,6 @@
 Binary Classification Metrics Example.
 """
 from __future__ import print_function
-import sys
 from pyspark import SparkContext, SQLContext
 # $example on$
 from pyspark.mllib.classification import LogisticRegressionWithLBFGS

examples/src/main/python/mllib/gradient_boosting_classification_example.py

@@ -20,8 +20,6 @@ Gradient Boosted Trees Classification Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext
 # $example on$
 from pyspark.mllib.tree import GradientBoostedTrees, GradientBoostedTreesModel

examples/src/main/python/mllib/gradient_boosting_regression_example.py

@@ -20,8 +20,6 @@ Gradient Boosted Trees Regression Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext
 # $example on$
 from pyspark.mllib.tree import GradientBoostedTrees, GradientBoostedTreesModel

examples/src/main/python/mllib/random_forest_classification_example.py

@@ -20,8 +20,6 @@ Random Forest Classification Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext
 # $example on$
 from pyspark.mllib.tree import RandomForest, RandomForestModel

examples/src/main/python/mllib/random_forest_regression_example.py

@@ -20,8 +20,6 @@ Random Forest Regression Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext
 # $example on$
 from pyspark.mllib.tree import RandomForest, RandomForestModel

examples/src/main/python/mllib/recommendation_example.py

@@ -20,8 +20,6 @@ Collaborative Filtering Classification Example.
 """
 from __future__ import print_function
 
-import sys
-
 from pyspark import SparkContext
 
 # $example on$

examples/src/main/python/streaming/sql_network_wordcount.py

@@ -29,7 +29,6 @@
 """
 from __future__ import print_function
 
-import os
 import sys
 
 from pyspark import SparkContext