[SPARK-20555][SQL] Fix mapping of Oracle DECIMAL types to Spark types in read path

## What changes were proposed in this pull request?

This PR is to revert some code changes in the read path of https://github.com/apache/spark/pull/14377. The original fix is https://github.com/apache/spark/pull/17830

When merging this PR, please give the credit to gaborfeher

## How was this patch tested?

Added a test case to OracleIntegrationSuite.scala

Author: Gabor Feher <gabor.feher@lynxanalytics.com>
Author: gatorsmile <gatorsmile@gmail.com>

Closes #18408 from gatorsmile/OracleType.