[SPARK-14456][SQL][MINOR] Remove unused variables and logics in DataSource

## What changes were proposed in this pull request?

In DataSource#write method, the variables `dataSchema` and `equality`, and related logics are no longer used. Let's remove them.

## How was this patch tested?

Existing tests.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #12237 from sarutak/SPARK-14456.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSource.scala

@@ -348,16 +348,6 @@ case class DataSource(
         PartitioningUtils.validatePartitionColumnDataTypes(
           data.schema, partitionColumns, caseSensitive)
 
-        val equality =
-          if (sqlContext.conf.caseSensitiveAnalysis) {
-            org.apache.spark.sql.catalyst.analysis.caseSensitiveResolution
-          } else {
-            org.apache.spark.sql.catalyst.analysis.caseInsensitiveResolution
-          }
-
-        val dataSchema = StructType(
-          data.schema.filterNot(f => partitionColumns.exists(equality(_, f.name))))
-
         // If we are appending to a table that already exists, make sure the partitioning matches
         // up.  If we fail to load the table for whatever reason, ignore the check.
         if (mode == SaveMode.Append) {