[MINOR] [SQL] Removes an unreachable case clause

This case clause is already covered by the one above, and generates a compilation warning.

Author: Cheng Lian <lian@databricks.com>

Closes #6214 from liancheng/remove-unreachable-code and squashes the following commits:

c38ca7c [Cheng Lian] Removes an unreachable case clause

sql/core/src/main/scala/org/apache/spark/sql/sources/rules.scala

@@ -101,7 +101,6 @@ private[sql] case class PreWriteCheck(catalog: Catalog) extends (LogicalPlan =>
           }
         }
 
-      case logical.InsertIntoTable(LogicalRelation(_: InsertableRelation), _, _, _, _) => // OK
       case logical.InsertIntoTable(LogicalRelation(_: HadoopFsRelation), _, _, _, _) => // OK
       case logical.InsertIntoTable(l: LogicalRelation, _, _, _, _) =>
         // The relation in l is not an InsertableRelation.