SPARK-2001 : Remove docs/spark-debugger.md from master

Per discussion in dev list:
"
Seemed like the spark-debugger.md is no longer accurate (see
http://spark.apache.org/docs/latest/spark-debugger.html) and since it
was originally written Spark has evolved that makes the doc obsolete.
There are already work pending for new replay debugging (I could not
find the PR links for it) so I
With version control we could always reinstate the old doc if needed,
but as of today the doc is no longer reflect the current state of
Spark's RDD.
"