[SPARK-19029][SQL] Remove databaseName from SimpleCatalogRelation

### What changes were proposed in this pull request?
Remove useless `databaseName ` from `SimpleCatalogRelation`.

### How was this patch tested?
Existing test cases.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #16438 from gatorsmile/removeDBFromSimpleCatalogRelation.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala

@@ -573,7 +573,7 @@ class SessionCatalog(
         val view = Option(metadata.tableType).collect {
           case CatalogTableType.VIEW => name
         }
-        SubqueryAlias(relationAlias, SimpleCatalogRelation(db, metadata), view)
+        SubqueryAlias(relationAlias, SimpleCatalogRelation(metadata), view)
       } else {
         SubqueryAlias(relationAlias, tempTables(table), Option(name))
       }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/interface.scala

@@ -318,7 +318,6 @@ trait CatalogRelation {
  * Note that in the future we should consolidate this and HiveCatalogRelation.
  */
 case class SimpleCatalogRelation(
-    databaseName: String,
     metadata: CatalogTable)
   extends LeafNode with CatalogRelation {
 
@@ -335,8 +334,4 @@ case class SimpleCatalogRelation(
       }
     dataCols ++ partCols
   }
-
-  require(
-    metadata.identifier.database == Some(databaseName),
-    "provided database does not match the one specified in the table definition")
 }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalogSuite.scala

@@ -433,24 +433,24 @@ class SessionCatalogSuite extends SparkFunSuite {
     sessionCatalog.setCurrentDatabase("db2")
     // If we explicitly specify the database, we'll look up the relation in that database
     assert(sessionCatalog.lookupRelation(TableIdentifier("tbl1", Some("db2")))
-      == SubqueryAlias("tbl1", SimpleCatalogRelation("db2", metastoreTable1), None))
+      == SubqueryAlias("tbl1", SimpleCatalogRelation(metastoreTable1), None))
     // Otherwise, we'll first look up a temporary table with the same name
     assert(sessionCatalog.lookupRelation(TableIdentifier("tbl1"))
       == SubqueryAlias("tbl1", tempTable1, Some(TableIdentifier("tbl1"))))
     // Then, if that does not exist, look up the relation in the current database
     sessionCatalog.dropTable(TableIdentifier("tbl1"), ignoreIfNotExists = false, purge = false)
     assert(sessionCatalog.lookupRelation(TableIdentifier("tbl1"))
-      == SubqueryAlias("tbl1", SimpleCatalogRelation("db2", metastoreTable1), None))
+      == SubqueryAlias("tbl1", SimpleCatalogRelation(metastoreTable1), None))
   }
 
   test("lookup table relation with alias") {
     val catalog = new SessionCatalog(newBasicCatalog())
     val alias = "monster"
     val tableMetadata = catalog.getTableMetadata(TableIdentifier("tbl1", Some("db2")))
-    val relation = SubqueryAlias("tbl1", SimpleCatalogRelation("db2", tableMetadata), None)
+    val relation = SubqueryAlias("tbl1", SimpleCatalogRelation(tableMetadata), None)
     val relationWithAlias =
       SubqueryAlias(alias,
-        SimpleCatalogRelation("db2", tableMetadata), None)
+        SimpleCatalogRelation(tableMetadata), None)
     assert(catalog.lookupRelation(
       TableIdentifier("tbl1", Some("db2")), alias = None) == relation)
     assert(catalog.lookupRelation(