[Doc][GraphX] Remove Motivation section and did some minor update.
[Doc][GraphX] Remove unused png files.
[SPARK-4472][Shell] Print "Spark context available as sc." only when SparkContext is created...

... successfully

It's weird that printing "Spark context available as sc" when creating SparkContext unsuccessfully.

Author: zsxwing <zsxwing@gmail.com>

Closes #3341 from zsxwing/SPARK-4472 and squashes the following commits:

4850093 [zsxwing] Print "Spark context available as sc." only when SparkContext is created successfully