Remove unicode operator from RDD.scala

Some IDEs don’t support unicode characters in source code. Check if this breaks binary compatibility.

Author: Doris Xin <doris.s.xin@gmail.com>

Closes apache#1119 from dorx/unicode and squashes the following commits:

05618c3 [Doris Xin] Remove unicode operator from RDD.scala

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -446,7 +446,7 @@ abstract class RDD[T: ClassTag](
    * Return this RDD sorted by the given key function.
    */
   def sortBy[K](
-      f: (T) ⇒ K,
+      f: (T) => K,
       ascending: Boolean = true,
       numPartitions: Int = this.partitions.size)
       (implicit ord: Ordering[K], ctag: ClassTag[K]): RDD[T] =