Remove mandatory fields for BaseFileOnlyRelation

Author: Alexey Kudinkin

hudi-spark-datasource/hudi-spark-common/src/main/scala/org/apache/hudi/BaseFileOnlyRelation.scala

@@ -54,9 +54,7 @@ class BaseFileOnlyRelation(sqlContext: SQLContext,
 
   override type FileSplit = HoodieBaseFileSplit
 
-  override lazy val mandatoryFields: Seq[String] =
-  // TODO reconcile, record's key shouldn't be mandatory for base-file only relation
-    Seq(recordKeyField)
+  override lazy val mandatoryFields: Seq[String] = Seq.empty
 
   override def imbueConfigs(sqlContext: SQLContext): Unit = {
     super.imbueConfigs(sqlContext)