Remove now-redundant special case from Array#lastIndexOf.

After the recent bugfix, the special case for 'undefined' is no longer
needed.

Bug: v8:7813
Change-Id: Iee3fccd72c525ac86a6fa6b3c55bcd2ce8159852
Reviewed-on: https://chromium-review.googlesource.com/1161906Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54897}

src/js/array.js

@@ -853,16 +853,8 @@ DEFINE_METHOD_LEN(
       }
     }
     // Lookup through the array.
-    if (!IS_UNDEFINED(element)) {
-      for (var i = max; i >= min; i--) {
-        if (i in array && array[i] === element) return i;
-      }
-      return -1;
-    }
     for (var i = max; i >= min; i--) {
-      if (i in array && IS_UNDEFINED(array[i])) {
-        return i;
-      }
+      if (i in array && array[i] === element) return i;
     }
     return -1;
   },