Remove the "experimental" warning on SlidingPaneLayout

Docs-only change. SPL is shipped in Hangouts and here to stay!

Change-Id: I6893a74d24aba50f7e3afdef2e7d8c3d44e10faf
diff --git a/v4/java/android/support/v4/widget/SlidingPaneLayout.java b/v4/java/android/support/v4/widget/SlidingPaneLayout.java
index 649a0cc..aaed38f 100644
--- a/v4/java/android/support/v4/widget/SlidingPaneLayout.java
+++ b/v4/java/android/support/v4/widget/SlidingPaneLayout.java
@@ -84,8 +84,6 @@
  * sized to fill all available space in the closed state. Weight on a pane that becomes covered
  * indicates that the pane should be sized to fill all available space except a small minimum strip
  * that the user may use to grab the slideable view and pull it back over into a closed state.</p>
- *
- * <p>Experimental. This class may be removed.</p>
  */
 public class SlidingPaneLayout extends ViewGroup {
     private static final String TAG = "SlidingPaneLayout";