Remove unnecessary code

This was added before we removed the call to `setJars` in SparkILoop.
Back then, `spark.jars` contained the "C:/foo/bar" paths, because we
called `setJars` on these paths. After we removed the call to
`setJars`, `spark.jars` contains the "file:/C:/foo/bar" paths, in
which case the downstream code does not need to be changed.

Long story short, `SparkILoop#getAddedJars` is only used for setting
class paths on the driver side as of this PR.