Fixed compilation.

Author: rxin

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -24,7 +24,6 @@ import org.apache.spark.annotation.Experimental
 import org.apache.spark.sql.catalyst.ScalaReflection
 import org.apache.spark.sql.catalyst.analysis.{UnresolvedFunction, Star}
 import org.apache.spark.sql.catalyst.expressions._
-import org.apache.spark.sql.catalyst.expressions.mathfuncs._
 import org.apache.spark.sql.types._
 import org.apache.spark.util.Utils