[SPARK-16732][SQL] Remove unused codes in subexpressionEliminationForWholeStageCodegen

## What changes were proposed in this pull request?
Some codes in subexpressionEliminationForWholeStageCodegen are never used actually.
Remove them using this PR.

## How was this patch tested?
Local unit tests.

Author: Yucai Yu <yucai.yu@intel.com>

Closes #14366 from yucai/subExpr_unused_codes.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -662,10 +662,6 @@ class CodegenContext {
     val commonExprs = equivalentExpressions.getAllEquivalentExprs.filter(_.size > 1)
     val codes = commonExprs.map { e =>
       val expr = e.head
-      val fnName = freshName("evalExpr")
-      val isNull = s"${fnName}IsNull"
-      val value = s"${fnName}Value"
-
       // Generate the code for this expression tree.
       val code = expr.genCode(this)
       val state = SubExprEliminationState(code.isNull, code.value)