[SPARK-17301][SQL] Remove unused classTag field from AtomicType base class

There's an unused `classTag` val in the AtomicType base class which is causing unnecessary slowness in deserialization because it needs to grab ScalaReflectionLock and create a new runtime reflection mirror. Removing this unused code gives a small but measurable performance boost in SQL task deserialization.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #14869 from JoshRosen/remove-unused-classtag.