2nd stage of split semantics change: remove deprecated String.split(String), rename splitBy back to split.

libraries/stdlib/test/collections/CollectionTest.kt

@@ -489,7 +489,7 @@ class CollectionTest {
     }
 
     test fun decomposeSplit() {
-        val (key, value) = "key = value".splitBy("=").map { it.trim() }
+        val (key, value) = "key = value".split("=").map { it.trim() }
         assertEquals(key, "key")
         assertEquals(value, "value")
     }