Revert "Added logging for serialization / deserialization."

This reverts commit a5d1e5b76c51c8dce9e37b3fb3835489edc367fd.
Logging for serialization / deserialization is not especially useful
because serialization and deserialization time is too small relative
to the overhead of logging (deserializing each record takes about 1us).
To understand how much time is spent doing (de)serialization, we should
use YourKit.

Conflicts:
	core/src/main/scala/org/apache/spark/executor/TaskMetrics.scala

Conflicts:
	core/src/main/scala/org/apache/spark/scheduler/JobLogger.scala